I don't know if you've found the problem yet, but if your switch was working perfectly fine and just quit engaging, it might be your TCCM (transfer case control module). If your transfer case switch doesn't light up when you start the truck or when your driving, its most likely a bad tccm. It's basically a computer that can tell if your truck is ready to make a shift into 4X4 or not. It's located way up under the dash and you can pick one up at a scrap yard for around 150$ or order it new from the dealer for 400$. Mine went out on my 02 QC Sport after I replaced the throttle position sensor and disconnected the battery.
4.7L V8, Superchips 3865, and a bunch of others. Just look at my garage.
"When people say I drive a small truck, I simply respond that I'm compensating for my large penis."