It's apparent from your explanation the vibration may be caused by a power link (half shaft) front or rear wheel bearing or rear axle.
Mu favorite way to chase down vibrations is to jack the frame on jack-stands, start the vehicle and shift it into low gear. Watch the tires and wheels for lateral and circular run-out as they rotate. Watch the spin of the half shafts and drive-shaft. If you don't see anything out of the ordinary, have someone slide the transmission into high gear and slowly increase the speed. You watch for a specific vibration.