vibration at 25-40 mph

So I put the car on jackstands and left it idling in Drive while I looked under it. Not surprisingly, the driveshaft was wobbling quite a bit. I got it to wobble a lot less by repositioning the rear u-joint in the yoke. I made sure the bearing caps were pushed inboard so there's no side-to-side play in the u-joint. That didn't really affect the vibration while driving though. Also, now I have differential chatter. At least that's what I think it is. I had it right after installing the 8 1/4 rear, and it went away after I added a bottle of friction modifier. But now that I adjusted the u-joint, it's back. Any ideas?

The u-joints are brand new and they didn't seem to have any problems before I installed them.

As to the other ideas: I'll try to check the pinion angle, but I'm reluctant to get the front end aligned because I'm just about to rebuild it anyway. I just would like to make sure the rear is squared away before I tear into the front.