4x4 lights won't come on and won't shift into 4x4

Start diagnostics with a fully functional scan tool and try to retrieve codes. The module powers up via a 15 amp 4WD fuse in the dash panel and a 30 amp Trec fuse in the under hood fuse block. Sometimes the switch illumination fails and that doesn't actually disrupt the 4WD system operation, that's why you need to start with a scan tool, preferably a Tech II.