2000 CLK430 Terrible Tire Cupping!

[mbs xk8]

Hello all! I've got a terrible sound coming from the rear of my CLK Convertible. It starts out as a whooom, whooom, and turns into a very loud hummmm at highway speeds. It's so loud the rear seat passenger can't hear the conversation going on in the front seats!

I've just replaced all 4 tires and got an alignment, but it didn't get rid of the noise. The rear tires had cupping issues on the inside edge. The right rear tire had cups that exposed the metal belt in the tire.

My question is... can the wheel bearings on the right and left rear wheels go out at the same time? Or perhaps I have another issue? Can the differential drop down to where it causes the rear wheels to ride on the inner edges of the tires?

I really appreciate any guidance you can provide! Oh, btw - the car is a 2000 CLK with 117,000 miles.