I have a 1995 chevy 3500 HD with a 6.5 turbo diesel. There is 23,678 miles and has a 15000 GVW. The truck was used by the NY transit authority to steam clean bus stops in NY city for ten years. This truck carried about 6 ton all the time when it was in service. I now have this truck and carried about 2000 pounds on it. My problem is when i am going about 45 miles per hour and let off on the gas pedal slow there is a vibration in the drive shaft ONLY when there is no load on the drive train. I have change all the U-joints 3 total. I also changed the center support bearing. I went as far as chiming the support bearing down thinking the drive shaft had to much pitch(over 7 degrees). The differential is a dana 80 with a 4.63 and a locker rear end. I did have the 2 drive shafts spun balanced to. Can anyone tell me where to look next to fix this problem?? ??