Well, starting to suspect carrier bearing. It would appear there is a rubber bushing surrounding it, which seems a bit odd, but then so does everything about this driveshaft setup. When I spin the driveshaft very slow I can see a little radial movement in the bearing. Seems odd that indexing the shaft would make it better. The vibe is just almost gone... Almost... Guess I'll be looking for a new bearing. Hope it's not ungodly expensive or hard to find.