2005 Chevy Tahoe service 4WD/stability codes

Tahoe has service 4wd and auto stabilty message flashing across dash display. Selector switch on dash will do a light test and then won't llight up 2wd or 4wd. Check fuse, replaced selector switch, nothing. Assuming it's a sensor or switch somewhere? Won't switch to 4wd and transmission won't shift down at start, just stays in drive.