If the driveshaft feels solid (no slack or play), there isn't any gear oil leaking at the bearings (it usually slings out all over the inside of your tire), and there is the proper amount of oil in the diff housing, then maybe you can jack it up, start it, put it into gear and see if you can isolate the sound while the wheels are spinning in the air. Mind you, this is potentially one of those "hey, watch this" moves, so be very careful. Or, if you have somebody you don't like much, you can have them hang off the rear bumper while you are driving down the road. Just kidding, but seriously, this is one of those noises that only happens while you drive and can be hard to figure out, until it gets a lot worse, or breaks altogether. Just try to isolate where the sound is coming from exactly, and diagnose from there.