I have a 2002 GMC Envoy that locked into 4wd a few mnths ago and would not unlock. So the first thing I was told was change the selector switch. Did that and nothing happened. Then learned the transfer case motor was bad. Replaced it and it kicked back to 2wd, it made the "service 4wd light" come on the dash and I cant get any other option on the selector, it actually lights up all four options when I start the Envoy then all the lighted dots go blank and the vehicle stays in 2wd. I just bought a transfer case switch, but dont know where it goes. Any help or direction would be really appreciated.