Drive Train Vibration???

[Smitty1]

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?? ??

[Axle]

I have experienced drive train vibration on older Chevy 1/2 ton truck I owned. It was caused by a few loose bolts on the automatic transmission bell housing. The vibration only happened at certain speeds and loading. It was some kind of harmonic vibration. I went through a lot of the same steps that you did trying to resolve the vibration. Then one day I decided to check all the bolts and to my surprise discovered some that were loose. Tightening all the bell housing bolts resolved the vibration completely.