Are you running factory sized tires? Or big oversized jobs. Have they tried turning the driveshaft 1/2 turn in the diff? I had a bounce in my 2500 4wd. I'm a tech and balanced the rear tires myself, still had noticeable to me vibration. I took tires off and match balanced by turning tire on rim per the balancer suggestion. It took the majority of the vibration out. They are used tires that have been on the ruck. So I'm OK with it now.