I have owned a 72, 2 76s, a77, and 2 79s, and I never checked the 72, but all the others were rated at around 1400 lbs, and I loaded them with that much several times. Most half-ton pickups are rated at more than a half ton. I think that is the minimum that they will carry.
You made a mistake, unusual for you! 620 load capacity was rated as 1400 lbs in the USA, as someone who has owned 6 of them over the years, I know that well.
That is how stupid our government is. That expensive truck didn't have spring parking brakes, which all highway trucks have been required to have since the 1970s.
The damper will only mask the problem. You have wear somewhere, or an alignment problem causing the death wobble. your safety is at issue here along with others, so fix the problem, don't just put a bandaid on it. When I had my Jeep many had the dreaded death wobble and the best cure was an aftermarket track bar that was better quality than what the cheapo Chrysler factory part. Your van doesn't have a track bar, but something in your steering or suspension has slop in order to cause the death wobble.
Back when I was a youngster my grandfather called it shimmy and that meant a trip to the alignment shop.