I also have a 2000 Bravada with 107k mi in which the check AWD light came on. I tried to put a scanner onto the port, but found no reading. There is a 10A fuse for the computer and a 20A fuse for the entire AWD system. If the 20A is blown, it will take the computer down with it. This was the case in mine. I replaced the encoder and the AWD was active again. But now the clutches in the transfer case were binding, so I flushed it and added the new spec fluid. I then drove it in a parking lot under 5mph turning the wheel lock to lock until it was freed up. Then changed the fluid again. The AWD system is now nice and responsive again.
Source: I didn't read this somewhere; This is cold hard logic and experience.