GISguy

Between 30-40mph I experience a heavy, roaring/rumbling vibration somewhere under the rear seats/center console. I've changed transmission mount and the problem remains. (although the mount pretty much eliminated any vibration I had at idle and made gearshifts much smoother)

Could it be an unbalanced wheel or two in the rear? This has been tested and repeated with the rear-end on jackstands with the traction control off, so I'm thinking it's not that, though I could be wrong.