Matt, FWIW, I have 271,000 on my '92 325i manual and I have never changed the trans or rear end oil and am still on my original wheel bearings, cv joints and clutch.
Hopefully it is just your tires!
Wheel bearings would tend to make sense if the noise was not present when run off the ground like you stated though.
The only noise I ever heard from the rear end was due to a frozen up parking brake mechanism (we rarely use it). I just put new rear pads on last month though and while I was in there, I freed up and lubed the parking brake shoe activator, so that little issue is gone for awhile. It did not click, you could just hear a rub and maybe an odd noise when when starting out or reversing direction.