The transfer case in the Dodge Durango has multiple gear ratios, which can be selected by moving the switch in the cabin to 2Hi, 4Hi, or 4Lo. It has been known to shift to 4Lo without warning, at any speed, and whether the vehicle is on or off. If this occurs at highway speeds, the result can be catastrophic failure of the transfer case.

This is caused by a faulty four wheel drive shift motor, selector switch (in the cab) or the wiring between the two. The most common cause is the transfer case switch, which is an electric motor mounted on the transfer case. 

To correct the issue, inspect the wiring to the transfer case switch motor for damage, and replace the shift motor if needed. Disconnecting power to the shift motor will lock the transfer case in its current setting.