[Roadtrip]

Have you checked that you don't have a bent wheel? Put the car up on securely on jackstands or a hydraulic lift and spin the wheels slowly looking for run-out. If safe and solidly secure to do so, you might even run the gear-train up and see if the vibration is still there, then take the rear wheels off and note any difference. How about the rubber dounuts (guibos) and driveshaft center support bearing/carrier? Are you sure of their integrity?

It may sound stupid, but check the exhaust system to see if it's rubbing against anything. Vibration is a funny thing . . . . sometimes it's coming from where it doesn't make intuitive sense.

Very bad move not to check the clutch while you had the transmission out anyway. Let's eliminate the easy-to-check stuff before we get into the hard stuff.