Billy, I've been under the VX and looked at the drive shaft, although admittedly not THAT closely. It's still round, but I suppose one of the balancing weights could have fallen off of it. I didn't hit it on anything so I can't see how it would be bent. With the VX in neutral there is no slop in the u-joints, and they have been recently greased. Yes there is a vibration that you can feel in the floor and the seat of your pants when the noise happens. The noise and vibration only happen in third, forth and over drive and only at highway speeds (55 mph and above). I would think a drive shaft imbalance would show up at lower speeds too.

Luna-X, the transmission, front and rear diffs and the transfer case fluid was all changed about 30k miles ago with Amsoil fluids, so the should (I stress "should") be good to go. The Amsoil is supposed to be some kind of crazy milage fluid.

Thanks guys, I'll keep plugging away at it.