1998 and newer Blazer, Jimmy and pick ups had 2 choices: NV 233 (selectable) 2 speed, and NV 136 (Automatic) single speed. Both cases are electric shift. I think Bravada had the NV 136 only. I can't remember (for certain).
A capable scan tool is necessary to retrieve DTC's on the NV 136 only. To retrieve codes on the NV 233, pin #13 of the data link connector is grounded, the ignition turned on, then the codes blink on the switches. That's why we need to know the model number first. I'm guessin' it's the NV 136 because he said the AWD light is on, and AWD is the NV 136, but need to know for sure.