Thanks for the reply Doc - the problem is that the shudder is occurring, and is actually worse, with the 4matic system shut off in test mode. My understanding is that the hydraulic unit is shut down in this mode, as you also lose the self leveling rear suspension. Otherwise I agree with you.
I've heard that MB made changes to the 4matic system every year between 91' to 93'. Any chance that the ASD system was made to operate independently from the 4matic? I keep coming back to the other constant which is the steering angle. It would all makes sense if one of the front sensors was sending a bad signal to the hydraulic unit, and engaging the diff lock. I was thinking about disconnecting the ASD control unit as a test, but Alldata doesn't have a good pict of the location, and there are several control units in that area.Any ideas?