Rear end vibration

Improving your leaf springs will stop the axle wrap and the vibration will go away. I'm thinking his springs are done.

Not sure why his springs would be done on a 2015. Maybe overloaded the truck? Either way he needs to take it back to a different dealer and get the problem resolved.

 

Are the rear tires worn unevenly? Shouldn't need to, but have you checked to make sure there is plenty of diff fluid?

Torquing the u-bolts was mentioned. Minimum 75 ft-lbs. This does help. As I know that mine have loosened in the past after some work, and the rear started behaving unnaturally.

Better leaf springs was mentioned. When I swapped from my '06 4x4 3 pack, to the stiffer springs from an '08 XR, it was no longer required for me to drive with the traction bars. Traction bars were required with the original leaf packs. Traction bars were eventually re-installed due to racing necessity.

Fine tuning the rear pinion angle also greatly resolved the rest of the oscillations from the rear. As the u-joints were moving at slightly different speeds. Causing one to speed up and slow down. Which translated into a slight rear end "hop" or "bounce." Notice at slow speeds on flat surfaces as well as at higher speeds.