Stock trans mount/height??solid/poly mount?? Maybe the rear axle housing bent.any problems sliding the axles in/out?? Posible bent spring/making thrust angle off.rear wheels pointing more 1 way then the other.(dog tracking) Do you buzz the engine to the same rpm in 1st and 2nd?? Switching to 3.23 gears made the vibration come at a higher mph,Then It has to be rpm related A shot in the dark.the fan??? A cracked flywheel will usually make some noises tailshaft bushings ok?? Vibrations can travel.What if the vibration is up front of the shaft,but the rear u-joints go bad because there is no room to move/ more tightly held together. Have you had it on jackstand,and whatched the driveshaft,should be pretty smooth,any osilation. just thinking outloud.
