Driveline vibration out of now where! Help!

AJ/FormS said

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?

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Yes, checked them both, no binding, no slack, nothing..I wish i could crawl under it while someone ran it through the gears so i could check it out, but no way im doing that! There has got to be a way to track this down better..like i said, with the driveshaft disconnected from the rear, and then spinning the rear wheels or pinion, there isnt any noise, but we cant spin them that fast either, but after installing the driveshaft and doing the same spinning the rear wheels, i can get under the tailshaft and hear/feel this..