If it were mine, I would jack up the rear and run it in gear using the brake to simulate the load to reproduce the noise. Using a long screwdriver to my ear I would listen to the Dana output bearing and to the 20 pinion bearings. The screwdriver acts like a stethoscope and you will hear where the noise is coming from. Or you can by an automotive stethoscope to do the same thing.
However, Having someone run it in gear while you crawl underneath is dangerous. Or, you could take it to a garage and have them do it on a lift.
My guess is ring or pinion bearings.