I had the same problem (vib after 75 or so) and started chasing the wheels. Turns out it was my driveshaft. Part of it was when the driveshaft was made, part of it was the angles. The U joints were not aligned (had an old guy make it who was winding down his driveshaft shop after 60 years in the biz and I guess he isn't as sharp as he used to be). Pulled the drive shaft and took it to another shop to check the balancing who informed me of the U joint alignment. They corrected it and rebalanced it.
Anyway, after installing it I measured the angles and had to raise up the nose of the diff 1/4 of an inch to get it to 2 degrees difference from the trans/shaft and with the diff/shaft.
So check your angles is my advise. Get the trans flange within 2 degrees of the driveshaft and do the same with the diff/driveshaft. I just got mine back together yesterday, took her up to 115 and smooth as glass now! I couldn't be happier.
So, my money is on the drive shaft angles on where you vibration is coming from.