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Looking for a rear 4x4 differential

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I had a lot of noise in the rear and a lot of vibration in my rear end. U joints were definitely shot but after replacing them and new shocks and bushings the vibration is more aggressive. Speeds above 45, especially under excelleration. It would cruise around pretty good before (besides the noise) with a decent vibration between 45-55 mph but this too rough to be scooting around.

I’m already In way over my head 😆  but I’m thinking rear diff?

 

I’m looking for a rear differential for my 4x4 85 720. I found a full rear axle at the junkyard for 250, but buying one from these goombahs makes me uneasy. What do you guys think?

anyone in the northwest have one they would be willing to part with?

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You have to know what gearing you need, there is a plate in your engine compartment on the passenger side that on the bottom right has a number like 38, 41, 43, 46, what is your number?

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You may want to check the sliding spline connection located just ahead of the rear diff before you change out the diff. Also, try jacking up the rear end and rotating the tires, listen for diff noises. If you can get someone to do that while you look and listen it may be a cheaper problem elsewhere. Have you checked transfer case and crossmember mounting bolts? 

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Make sure it's the rear end first by eliminating the following. I've never seen a differential cause much vibration.

Vibration causes...

 

Drive shaft balance weight fell off. Inspect for new rust spot. Usually a high speed vibration

Drive shaft damaged and is bent. Look for dent or scrape marks. Much stronger vibration.

Balance weight fell of rim. Tire out of balance. Swap to front one at a time see if vibration moves to the front. Lower frequency vibration and stronger again

Tire damaged? has bulge in tread or tread separation, other. Same as above.

Brake drum broken and metal missing. (had this once)

 

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I buy used parts from wrecking yards all the time, and there's not a lot to be afraid of with a used rear axle assembly for $250...as long as the diff is good and the gear ratio is correct.

 

Just make sure they give you some kind of guarantee. if you get the diff out and find that the gears are bad (or spider gears or bearing, etc) you should get your money back. Yards will do that if they want to keep their reputation.

 

Be cautious that the plate doesn't always tell you the gear ratio. Someone could have swapped in another ratio at some point. The best way to tell is to count the revolutions or even count the teeth on the gears.

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20 hours ago, wayno said:

You have to know what gearing you need, there is a plate in your engine compartment on the passenger side that on the bottom right has a number like 38, 41, 43, 46, what is your number?

It’s 41

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I appreciate these replies, real good info.

After I felt a bunch of play in my U joints I decided to take it to a shop and have them do it, and they said they replaced the u joints and that they failed on the test drive?!  When I went by to talk to them I noticed the drive shaft wasn’t in the truck.. is there a possibility the drive shaft balance weight fell off or was never put back on? It never had such bad vibration before, before it felt like an out of balance tire and a alignment was needed and there was noise when engaging gears that sounded like shot u joints. They said they replaced the cheap parts with spicer U joints but I’m still filled with regret for taking it into them. So ill need to do some investigating on my own when I have the time, and even though I don’t know Jack about diddly I won’t be taking it to a shop again. 🤬

 

I will start with switching tires around and see how that feels, thanks again everyone!

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C is a Salisbury type differential with the removable cover plate on the back. The differential ratio is stamped into the outer edge of the ring gear.  If 37:9 (divide them) you have your 4.11

 

Replacing the U joints will not entail removing the balance weights. They are welded on (if it needs it) at the factory. If you inspect the drive shaft you might see where one may have come off. The drive shafts are pretty close to balanced as is. Balancing them gets rid of annoying hums or droning sounds, they don't shake the truck unless damaged, dented or bent. 

 

Check the outside and inside edge of both tire rims for a missing wheel weight. Usually there is a clean spot. Or swap them to the front. While they are off look at the brake drum for missing metal. I had a 2" chunk of front drum fly off my '65 Pontiac. At 50 MPH the steering wheel was hard to hold onto. Ten MPH above or below was ok.

 

 

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First thing I would do now is remove the fill plug for the rear axle, stick my finger into the the hole and down into the gear oil, pull your finger out and look at what color it is, if anything other than golden clear I would remove the drain plug and empty/drain it and see what comes out, if any metal chucks come out buy another axle with 4.1 gears in it and install, fill with new fluid, you should be good to go then.

Yes you should check the gear ratio, the way I do it is lift one rear tire off the ground and put it in neutral with the e-brake off, block the front tires first.

Now mark the tire on the bottom sidewall straight down, mark the driveline on the side near where it connects to the rear axle, now slowly turn the rear tire/wheel 2 complete turns and count how many times the driveline turns, it should turn just slightly over 4 turns for 4.1 gears.

Now I have had others on here argue that the way I described above will not tell you the gear ratio, as long as the other rear wheel cannot move/turn, it will  work just fine, 2 complete turns of the wheel, count how many times the drive line turns, the C200 only has 4 different gear ratios, 4.1, 4.3, 4.6 or 4.8, so the drive line will turn one of these ratios, just over 4 turns, 4 and a third turns, just over 4 and a half turns, or just short of 5 turns.

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Just for an idea, if you dont mind swapping to something that has more parts available cheap and more ratios, the 80-85 Toyota 4x4 front axle and the 80-95 Toyota 4x4 rear axle are all 8" interchangeable 6 lug units that have tons of aftermarket parts still.

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They are maybe an inch wider than the 620. On a 4x4? pfft!

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21 hours ago, nl320what said:

Just for an idea, if you dont mind swapping to something that has more parts available cheap and more ratios, the 80-85 Toyota 4x4 front axle and the 80-95 Toyota 4x4 rear axle are all 8" interchangeable 6 lug units that have tons of aftermarket parts still.

That’s a nice to know, never would have thought of that. 

Well I crawled around underneath, and drained the differential oil. 

Drive shaft still has the balancing weight, and I didn’t see any missing weights on the wheels or bulging tires. (Still gonna get these balanced, who knows if those weights are helping or hurting me) 

Looked at the drums for missing pieces, but should I be looking inside? Never opened one up before.

As for the differential “oil” I’m not sure you could call it that anymore, it’s more of a sludge.. not sure how I will find metal shavings in it but I’m gonna find a strong magnet and maybe water it down with some gas to see and what I come up with. Any thoughts?

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Should be about 2 liters or 2 US qts. You should be able to dip your finger into the oil through the fill bung hole. The bottom drain has a magnet on it. If anything ferrous is loose in there it will stick to it. That 'soup' has water mixed in with it. This can be sucked in the differential vent from launching boats. (never submerge your differential) The 4x4 should have a rubber hose from the vent up to the under side of the bed to prevent this. The front one too. The cold water contracts the very hot air inside and sucks it in.  Or the pinion seal is bad. Grab the front differential U joint and try shoving it sideways. It should only turn, no other movement.

 

You don't have to take the drum off just turn it so you can look at it.

Are the wheel rims packed with dirt on the inside?  A friend slid his Pinto into the ditch sideways and packed clay in there. At 40 MPH the wheel was hopping off the pavement.

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On 11/30/2018 at 2:17 PM, nl320what said:

Just for an idea, if you dont mind swapping to something that has more parts available cheap and more ratios, the 80-85 Toyota 4x4 front axle and the 80-95 Toyota 4x4 rear axle are all 8" interchangeable 6 lug units that have tons of aftermarket parts still.

 1980 to 1985 Toyota 4x4 rear axles are 55" wide, after '85 to '95 they were about 58" wide and had much larger axle tubes. I use an early rear axle in my Rover and had to build a low profile truss to keep from bending axle tubes.

 

Some of these Nissan motorhome duallys had full flat rear axles so swapping in a Toyota axle would actually be a downgrade.

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Good to know about that soup. There was no metal chunks on the bung, but since it was so thick it could be holding some bits I can’t see, I’ll change the differential oil once more after driving it for a little while, see what I come up with.

I could be imagining things but it seemed to feel smoother and less violently shaking after replacing the differential oil..  I’ll have to go get an alignment and balance see if that makes a difference. Replacing the rear axle sounds daunting but after what I’ve done so far I’m determined to have this thing in top top shape.

 

Ok cool I’ll check that pinion when I get back home (in a little over a month 😑) in the mean time I’ll hope that the junkyard has the axle when get back, and really hope they still have 3 good fender flares when I call tm!

 

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Swap the rears to the front. See if the problem moves to the front.

 

You want GL-5 oil 80w90.

 

Alignment? That's for the front. Are you saying it's the front shaking? or the rear? Is the steering wheel shaking side to side?

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Good call I will have to move the tires up front before I spend any money.

I do think it’s needs an an alignment as well, it’s a little off. But I guess I was thinking of an taking it to a tire shop for alignment and balancing of the wheels all around, hoping balancing the rear wheels would help a the rear shaking a bit.

It shakes at about 45, and definitely more under acceleration. It is coming from the rear but the drivers side mirror also shakes in unison, but the passenger side mirror does not.

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4X4... you have a slip joint on the rear driveshaft. It's splined like a collapsible steering column so the driveshaft can compress as the suspension moved up and down. Grab the driveshaft and push and pull at right angles. If the drive shaft can be moved side to side it will be way out of balance when turning. It will wobble under load.

 

Next inspect the transfer case rubber mounts. Pry the case with a 2x4 and see if any are broken. The transfer case should be firmly rubber mounted.

 

Next look at the rear transmission rubber mount. Is it torn out?

 

Last look at the two engine rubber isolater mounts.

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39 minutes ago, datzenmike said:

4X4... you have a slip joint on the rear driveshaft. It's splined like a collapsible steering column so the driveshaft can compress as the suspension moved up and down. Grab the driveshaft and push and pull at right angles. If the drive shaft can be moved side to side it will be way out of balance when turning. It will wobble under load.

 

Next inspect the transfer case rubber mounts. Pry the case with a 2x4 and see if any are broken. The transfer case should be firmly rubber mounted.

 

Next look at the rear transmission rubber mount. Is it torn out?

 

Last look at the two engine rubber isolater mounts.

Awesome. Can’t wait to check all this stuff out!

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The FSM describes, for the 4x4 model, to connect the propeller shaft at the diff flange 180 degrees and take a test drive.  I'd mark the original location first.

 

Is it possible that the u-joint techs got the slip yoke 180 degrees, or just a spline off?

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 They should know better but you should be able to tell by looking at it.  I don't think the splines are made such that you can assemble them wrong. Nissan Pitman arms can only fit in 90 degree choices. Obvious as hell when you try to put them on.

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