It sounds like the viscous coupler/variable transfer clutch is “locked up”.
Why/how did this damage take place? By using tires of different sizes on the car.
The tire shop set up this sitation, due to their apparent ignorance of the importance of this issue on an AWD vehicle. The owner could have learned about it by reading the Owner’s Manual.
Probably the only fix is to replace the viscous coupler/variable transfer clutch (I don’t know which one is on this particular model), to the tune of…maybe about $600. However, the first step is to put 4 tires of the same size on the car.