My car has the stock alum. drive shaft. Before the car was pretty smooth except when you got to around 60-65 mph it would start to vibrate, and the one time I had it at the track, I noticed it would vibrate really bad on deceleration at the end of the track. I've also noticed that I was getting a lot of clunks when I put the car in reverse, so I changed the u-joints today. Now, the clunks are gone, but there is a vibration all the time now. I shouldn't need to rebalance the drive shaft after swapping u-joints should I? I thought maybe I messed up on the u-joint install. I bought the correct aluminum joints, and the front joint is loose, the rear one is a little stiff, but I can move it through it's entire range of motion ok.

Any ideas?

 

I'd start by getting it balanced. If you still have a vibe, check your pinion angle.