I worked with a local driveline shop to fix my 50 to 60 mph vibration problem
Here is the list of suggestions I followed to find the problem:
Put the rear axle on jack stands and in gear run the engine to the speed it vibrates.
Remove rear axles, retest, remove each driveline until vibration goes away.
Results were: the forward driveline extended by Gulfstream was not straight. I was told this is not an uncommon problem. Also tires will vibrate from sitting for long periods until they warm up after driving for some miles.