What's the symptoms both from a feel and sound perspective of a bad rear axle on a W124 wagon?

More importantly, how do I diagnose a bad rear-axle vs. bad rear diff.

I have a fast "waa-waa" sound that comes on between 45-80mph ONLY during accelleration.

I had the car on jack-stands, and could hear it (not feel it) both with and without wheels on. It sounds like from both the left & right sides. I felt a vibration in the rear-diff, but I don't know if that's normal or not.

FWIW, the rear bearings have been replaced about a year ago.

Would worn rear-diff bushings produce the same symptom (e.g., waa-waa sound on acceleration ONLY, goes away when lifting off the gas)?

TIA,
:-) neil
1988 360TE AMG
1993 500E