I see where you used three different driveshafts but did you have any one of them balanced by a machine shop?

are you absolutly sure the U joint fit's the rear yoke, had one on a car once that the U joint was a little narrower that the tab's in the yoke that center the U joint, and it was to one side more than the other and threw it off balance.

did you witness the rotation of the driveshaft when you had it on the stand's to make sure it was running true, I think total runout is supposed to be .010-.015 across the length of it.

that's all I can think of right now.