Like mentioned before, it is an electronic AWD system and relies heavily on the wheel speed sensors. It is not a traditional 4WD system. So, since the front spare tire was spinning faster than the normal tire on the front, the ABS system is trying to compensate and telling the AWD to put power elsewhere ie. the rear diff. Thus causing more stress on the rear and damaging it. Should've put the 50 cent fuse in and saved lots of trouble. I'd say the driver is 100% at fault.

The auto manufacturers give you an owner's manual, I suggest reading it.