Here is what the GM service manual sais about the code you have. I wonder if this code was created when you stopped the wheels when you had it jacked up. Just a thought.
DTC C0374
Circuit Description
The transfer case shift control module senses rear to front wheel slip, a difference of rear to front wheel speed and attempts to correct the difference by activating the encoder motor, to apply more torque to the front wheels.
Conditions for Setting the DTC
The transfer case shift control module is unable to correct sensed slip for 10 consecutive seconds.
Action Taken When DTC Sets
All transfer case shifting will be disabled while the DTC is present and current.
The diagnosis of this will need a scan tool, probably a GM scan tool. There are some special functions that need to be performed, and I don't know of the snap-on scan tool is cabable. I could post the instructions if you have a scanner that would do that. Hope this helps.