BCM has nothing to do with the 4wd system.
Clean all of the connections at the TCCM, switch, and encoder motor on the side of the transfer case.
Then test the switch for proper resistance values. Search for "nv236+resistance" and you should turn up the proper information. You can also search for "nv236" and get even more threads. Remove the quotes in both cases when you type that into the search tool.