I don't think you have enough travel. The shocks don't work if they aren't traveling enough to dissipate the energy. You are literally bouncing off the bump stops. Since the shocks don't have enough travel in them to absorb the impact you are hitting the bump stops so hard you are bouncing back off them, sending the rear end of the truck back up, probably pretty high... it comes back down so hard it hits the bump stops again and the cycle repeats... Need to shorten the bump stops so there is more clearance between them. This will create more suspension travel towards the body, make sure there is clearance else where for it.
