rearend woes

[nds03]

Ive been drinving on the interstate a lot lately and i noticed a vibration at high speeds. Every time Ive driven at high speeds it got even worse. Now i can feel them at like forty. The only time they go away is when im driving it and I let go of the gas while it is in gear. It is an 8 inch out of a 65 mustang. I think its worn out, but I'd like to be sure before I order new parts for it.
Any ideas?
-Noah

 

[Hack]

It could also be the u joints. I think you should check them first because they're cheaper and easier to fix.

 

[hdgapeach]

If it ain't the u-joints, check the level of oil in the rear axle. If it is good check the pinion bearing by pushing up and down on the pinion yoke to see if there is excessive movement. Next check would be the axle bearings. Put the rear on jack stands and push/pull on the tire like you would check the front for a bad ball joint. Up and down movement means bearings are shot or the housing is oversized and allowing the bearing to move. If it passes all the checks so far the ring gear and pinion or the transmission is the problem.