Drive Train Vibration

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?? ??

Howdy

Be sure the drive line angles are correct. All the angles need to cancel each other out.

So if the included angle at the tranny is 6 degrees then the angle at the next joint needs to be the same so the joints are running the same speed.
A u joint speeds up and slows down 4 times per revolution and it the front joint is at a different angle than the center and so on the things will do all sorts of wiggles.

Also make sure the joints are in phase(Looking at each other) if you have one out of phase the joints will be fighting each other and will really get hinky.

Its possible that the slip yoke is off a spline or two.
Also its possible that one of the shafts had a yoke replaced and the thing was welded on out of phase.

** All joints must be in line with each other. If they are off just a tiny bit they will shake rattle and roll.**

Angle is critical in as much as the joints at the diff must be of the ssame included angle as the ones farther back up stream.

General eaccepted practice is to keep things within 1 degree of each other.

Normally the rear diff pinion shaft will be paralell to the crank shaft of the engine and the drive lines will all fall in nicely with the angles.

The constant velocity joint found on many 4x4's came into favor to allow for the rules to be broken.

The double joint on the one end of the CV drive line cancel each other out so that matching the angles of the front and back joint combinations was not needed.

GM used the CV driveline on the front of many earlier rigs.

In many custom lifted trucks the use of a CV line will come into play to keep things smooth at speed.

Anyway, check the shafts to be sure all the joints are in phase and also that the angles are correct and compliment each other.

Hope this helps

Robyn

Thanks Robyn
If you have the drive shafts spin balanced should you still have this problem? When you spin balance the shafts should they be using the same angle as they are sitting in the truck? Also when you say the U Joints should be looking at each other what does this mean? I have three u joints and a center support bearing in this truck. Does the u joint looking at each other apply?