Rear diff, pinion bearing maybe? Although I've never had one go so not sure what to expect. Maybe even something to do with the mounts on the gearbox and the different load on them when in rear wheel drive? I had a bad vibration and it was the front driveshaft slip joint, it got to the point where I was going to stick a video camera under there and see if I could catch anything when I was trying to work out what it was, if you're desperate it may be an option.

Cheers
Steve