One common complaint with pickup truck owners is vibration while loaded up. The pinion angle is set with no load in mind, because that's the most likely scenario. When the suspension compresses, the pinion angle shouldn't change much, but it does change.
If you really want to fix the problem, you can get creative with a two piece shaft and a CV on the middle joint (like Toyota). Any driveline shop should be able to make you a custom setup, but you will have to verify or possibly reset the pinion angle to get the proper geometry.
I built a 63 Nova a couple years ago that rode on air bags. Since the customer wanted to be able to drive it at varying ride heights, we used a custom CV slip yoke on his driveshaft. No vibrations at all. I also use CVs on all of my offroad builds. It's just one of those details that gets overlooked by a lot of people.
These trucks come stock with a 2 piece drive shaft. I think he converted to 1 piece out of a 520, plus dropping does not help either. Its fixed now, just had to unload the bike. Merry Christmas.