That is indeed unusual speed for vibes from what i have read so far and experienced myself, typically 55 - 60 is when party starts.
As mentioned above, start with diagnosing rear, go through different angles in 0.5 increments. That should resolve or give you best angle that gives least amount of trouble. If you cant find angle that works, try rotating ds inside the yoke in tc and axle, that can resolve the issues.
If you have exhausted rotations and angles, take DS to a local drive line shop, have them check balance.
I am currently going through interesting situation with a new rear driveshaft, so far smoothest operation i got out of it at 0 degrees, which is a bit puzzling, but i am still vibrating/shaking past 55. Local shop told me that it was way out of balanced, so they decided to rebalance it without letting me know. Alright, cool, but i am still vibrating/shaking past 55.
From the transmission towards back everything has been replaced and renewed, there is nothing left to point fingers at besides the drive shaft. Will be sending it back to maker this week to see what they say.