Driveline vibration out of now where! Help!

I have never heard a pulsating boom-boom-boom from a tailshaft so cannot say.
But if it is the rear bushing, you should be able to reproduce it by manipulating the trans yoke in multiple directions, to find the loose spot. Now keep in mind that rear-end noises will travel up the driveshaft, so if you cannot find the looseness in the bushing, then.......... perhaps it doesn't originate there.
When you checked the rear U-joint, did you also check the front one?