Two piece drive shaft will shake like crazy if the slip joint is worn. One piece drive shaft will do the same. If you have checked the "U" joints and the carrier bearing I would look at the slip joint for wear. That allows a wobble to set up in the joint itself. That wobble is worse under load so you might not see it with the truck up off the wheels. We just did this on a 57 1/2 ton. Looked at all the other stuff. Wheel bearings, helped did not fix. Yep we did front and rear. New tires, helped did not fix. New shocks, all four, helped but did not fix. Grease gun on every darn zerk we could find...no joy. New "U" joints and a seal for the front yoke, no change. My buddy was not happy about the money spent and I was tired out. Final idea was the slip joint. Lucky for us we have a good shop in town they cut the old joint out and welded a brand new one in then balanced it. Truck is smooth strong and quiet up to 65 which is as far as the owner will push it. Good news is he has clean fluid in the rear and transmission that was really needed. He is now happy and I am out from under that truck. Good luck with yours.
