From all that was posted of the violent vibration , was starting to think you had an out of balance driveshaft or bend in it but now we see you have some detectable play in the pinion. Not likely it's only the outer bearing but if the worn u-joint caused some wear and didn't total the inner , you may get away with replacing the outer and pinion seal and re torquing but not a proper repair and can only buy some time. If not expensive , you might try this to see how long it lasts just to get an idea what condition the rear is in. But you may want to pull the inspection cover first to see how the ring gear is wearing . If noticeably worn , the inner bearing is most likely on its way out to . Key thing here is that the pinion should have no detectable play , like a front rotor or wheel bearing , it should be tight , just not binding. I would say to drop the shaft one more time from the rear and pull the cover to do a good inspection . Wouldn't be a bad idea at this point to get the wheels off the ground to take load off the side bearings and pry at the case to check the inner bearings while apart. Better to find now if you need a mini overhaul than to blow the rear and have to drive home on the front. Just a thought.
