Not to worry - you're not the first with this problem. I had a stock driveshaft out of phase for a while before I learned that it's a problem. I replaced it with a one piece shaft which removed much of my vibration at 60-70mph, but not all of it, even after rebalancing that one. Then, I bought a Shaftmaster's driveshaft which helped even more, but my vibration is still there. I'm sure my pinion bearings are the source, and need to check them this weekend (thanks IJ for how much play is allowed).I've heard that a lot. Why do I always have to be the first. :biglaugh:
Rebuilding my diff is on my short list now, but I've not gotten the parts yet.