"The first one is the AWD duty solenoid for applying and releasing the viscous couple when needed. The system needs to allow slip in the clutch when the front and rear tires are rotating at different speeds. This is usually only on turns."
That depends on the nature of the fault.the manufacturer has built in a default fail safe mode to allow you to limp along. In the case of a failing AWD solenoid one might think the fail safe would be to permanently release the viscous couple to only send power to the front wheels.