Going out on a limb here...
Do you actually feel the rear end bouncing, or is is purely an audible thing?
If it's just audible, then I'd suspect wheel bearings or other driveline bearings... can you feel the vibration any better with a hand on the transmission?... then maybe driveshaft phase/angle
If it's the whole truck bouncing, then I'd suspect the wheel/tire balance. Many tires, especially mud or all terrains can't be fully balanced with wheel weights.
One could put the rear on jack stands and see if you notice anything while the wheels turn.
