Generally, if the diff is the issue, the noise will change depending on load. It will likely get quieter on heavy loading and the noise will increase on cruise or at light loads. While this is not definitive, if you notice this behavior, then I'd lean toward the diff as the culprit.

The fact that you get nothing when the rear end is in the air would probably rule the diff out..........the load is minimal and it should be making a decent racket.

The fact that the noise disappears when the wheel bearings are unloaded also tends to point you in that direction.