Clunking on decel and cruising around 2500rpm

[TheBirdistheWord]

welded 3.9 r180 installed recently behind a close ratio ZX trans. I was 2 miles from home after a spirited drive on hwy 152 over the santa Cruz mountains. The clunking will entirely dissapate if I accelerate at any rate. I have seen a few related posts while searching and it seems the ujoints are the first thing to check.  All 4 half shaft ujoints have only 15k on them and are grease frequently, rear wheel bearings and CA bushings are new.  Mustache bar bushing replaced.  I think I can make a decent assumption it's in the shaft since it's literally the only thing I haven't serviced/replaced in the driveline/suspension.  

 

Is there a way I can verify driveshaft ujoints are culprit before I start pulling crap off and starting a goose chase? 

[datzenmike]

Lift the wheels in neutral and grab the driveshafts and push and pull hard on them. They should allow you to turn them but nothing else. No sideways (90⁰ to turning) motion what ever. Now do the transmission to differential drive shaft. Is the transmission mount tight? Mustache bar bolted down tight to differential? Differential bolted down tight? Lug nuts tight? Shock absorbers tight? Something rolling loose in the trunk? Exhaust pipe hitting rear cross member where it passes through? Loose muffler or clamp? Give the exhaust a good shove, does it hit anything?

[banzai510(hainz)]

Do what Mike said,but I would Check the half shafts closely again.Jack up car get under there and take one wheel and move back and forth while looking at the halfshaft U joints. A welded diff will be hard on the U joints and quality of them Especially a lowered 510.

If this is a 5speed C.K. the trans mount and bolts

the diff has those U shaped Washersmake sure your not missing  one.

[TheBirdistheWord]

Thanks for the suggestions!  Driveshaft has absolutely zero play when I yanked/rotated it for better part of 10 minutes.  Trans and diff and mustache mounts were tight. Just did some extensive work on exhaust and it is rattling on the xmemeber slightly but I know what that sounds like this vibration is new. Despite being quality replacement ujoints the driver side inner joint has some lateral play.  Kind of crazy I can get that kind of vibration from that little amount of play. Had a spare used one pressed in to verify and that was it.  Two days later head gasket on the tired l20 I had in there took a dump. *sigh*.  Universe telli g me to put my built l20 in there I guess.  I sense CV shafts in my future....

Edited December 31, 2019 by TheBirdistheWord

[datzenmike]

If there is any movement at 90 ⁰ to the axis of rotation this moves the driveshaft over, and while it seems like very little, that's a lot of metal weight off center.