I replaced the rear rotors and the shuddering totally disappeared. Many thanks to psfred - I would not have considered the rear rotors since the shuddering seemed to be coming from the front end. I would have bet my life on it but it was clearly a rear rotor problem.

I did re-check my instillation notes and the rear rotors were about 1 mm over the wear limit after resurfacing. I did find that the right rotor seemed to have quite a bit of brake dust inside the parking brake drum - maybe I adjusted the parking brake too tightly and the rear rotor heated up and warped?

At any rate the car stops smooth and straight once again - I won't be having any more MB rotors turned.

Tim