I have an appointment with the dealership to diagnose the situation. In dealing with this noise one of the difficulties I have had is in describing the noise. A groan? I guess it could be described that way. Another way is to imagine a croaking bullfrog caught under the car and each time the axle makes a rotation a muffled 'croak' come from under the car.
I did hear about a service bulletin regarding a rear end problem. Does anyone know the number of that bulletin? I downloaded the pdf list and bookmarked the site to read them.
I am really anxious to have this sound go away and return to quiet motoring.
PS - After writing about the M5 diff oil I realized that I am comparing apples to oranges and the M5 anecdote most likely does not apply to my situation.