It took a bit of time, labor and parts to dig this out on my truck. I had a driveline intermediate carrier bearing, rear u-joint and rear pinion seal replaced. Still bouncin'....
My problem was finally isolated to the tires even though several standard spin balances were done. I had a way out of spec tire on the back, which was replaced. The diagnostic was a road force balance, which isolated the source of the bounce and vibration.
How did I know to get a road force balance? I put a set of stock wheels and tires on the truck that had been checked with a road force balance and the truck ran as smooth as glass. (it was ugly lookin', though
Now it's all good. So see if you have a tire and wheel shop in the area that can do a road force balance.