I had a bad vibration at around 70mph in my mustang. After going through everything, even having the driveshaft balanced, the vibration was still there. In the end I finally figured it out, it turned out to be a bent output shaft in my tranny. Good luck with tracing it down. If you need to borrow a stock driveshaft, I think I still have mine, it might help you rule that out.

I might have to take you up on that K....i'll let you know something next week...thx!