The rear axle bearings are lubricated with wheel bearing grease, not the rear axle gear oil. Folks seem to never repack them. I have had a car make a sound like this and it was a dried up wheel bearing, the differential was just fine.

Running it on stands with no load on anything may not help you decide anything. Be very careful of the rotating tires and driveshaft when doing this test.

Removing the axle shafts is pretty easy thing to do to check the wheel bearings.