Drive Shaft U-Joint/Rear End Pinion Flange Problem

[rsatz28]

I need to resolve a problem that is causing a vibration under hard acceleration/high RPM. I’m not sure how to approach/resolve the problem.

When I purchased the car 10 years ago, the previous owner had installed some type of shim under the clamp on each side on the rear u-joints. When I did my resto work 4 years ago, I had new u-joints installed on the driveshaft. I was going to replace the rear end pinion flange, but was told not to do that myself do to the preload (I think) on the ring and pinion.

The problem is that is seems that the u-joints fit tightly in the rear end pinion flange, but after a short period of time I end up with side-to-side movement within the caps. The caps are not moving side-to-side within the yoke, but the part of the u-joint that the needle bearings are against inside the cap. The side-to-side movement is aprox. with caps in place is 1/8” to 3/16”. Does this make sense?

Before I purchase a new rear end pinion flange or question the new u-joint, does anybody know what the correct overall OD dimension of the required u-joint, or the ID of the “stops” on the rear end pinion flange? Or what else should I look for? Thanks