Rear Axle/Wheel Bearing/Diff Issue?

[Karacticus]

Removed the disks for replacement while the car's down for the winter, and started in on wirebrushing and putting a little paint on the hubs to keep the rust down.

While hand turning the rear hubs, noticed that there's a bit of a noise and vibration while driving the opposite side through the diff (3.0si Coupe, so open diff). It's moderately rapid (much, much more than 1/rev) so it's the sort of thing that might coincide with gears meshing in the diff or a bearing hitting a flat spot in a bearing race. Direction of rotation doesn't make any difference.

Boots on both axles are intact. I figure the next step is to put the car in neutral and fix one hub in place and see if it makes any difference turning one side versus the other.

Anyone else have any idea whether or not this is normal? Car is just shy of 100,000 miles.