I know a FEW members have had issues with the RDU, the first I heard of was glock-coma. Usually a whining or grinding sound is the giveaway. The viscous coupling/ATC portion of the RDU is sealed (I think filled with Motorcraft Mercon LV) has only been pinpointed as a problem in an Edge living in Russia IME. And no shuddering/chattering there either.
So I would check the wiring between the AWD/4x4 module and the ATC, and see if there are codes stored in the PCM first. Might be a wheel speed sensor or two at fault also. You can use AWD related PIDs to see what speed each wheel is turning at.