U Joint Cap Size

So I got them in...job went about like I expected, no real problems...easy, all things considered.

I still have a rhythmic vibration around 40mph and above....it does it if I put the car in neutral and shut down the engine and coast, so the engine is not doing it. My back tires are old as dirt so that's my next thing to change.

I guess the drive shaft could be out of balance.

One of the old U Joints had some play in it and I thought that was the vib....any other ideas?

Thanks for all the good help!

Brian