When it’s up on stands and you checking u-joints…. Check for rotational “play” AND end to end “play” in the pinion yoke. It only takes a few thousandths of an inch of slop to set up a harmonic vibration at just the right RPM range. The yoke on my car got slightly “splayed” during a u-joint change (DOH!) and I have that same type of vibration at around 65 MPH.