bad tranny mounts?

Kind of odd that it goes away between 50-54 mph, but from what you are describing, it sounds quite a bit like a twisted driveshaft. Many people on here have that problem. If you accelerate hard with your hand on the shifter, does it shake harder? The easiest way to check for a badly mangled DS is to put the rear end on jackstands (with the front wheels chocked) and have someone slip the car in gear and watch the DS under the car as the wheels turn. I'm also guessing the vibration gets worse the faster you go? Sounds like it might be time for a new aluminum DS; at $160 they are cheaper than getting your old one repaired and come with new u-joints.

What happens when you push in the clutch or drive the car in neutral at 55+ (whenever it shakes)?