You may have disconnencted wire/s running to the transfer case that realizes the truck is in 4X4 and sends no signal to the instrument panel. Is your truck a turn knob 4X4 selector like near the air vents or a stick in the floor?
In conclusion with mine not working..... It turned out to be the Control Module under the kickplate. When it started to go it would work when the truck was warm inside and then after a while not at all. I got a used one from a local auto junk yard and it's been been working great. I also had my battery go a while back like a few of you guys in posts above. When I put in my new CM (used) I had to give my transfur case motor a couple taps to get it working, now everything seems fine.
I put a 3"bodylift on over the summer and all car navigation system went good but now its snowing and my 4 wheel wont do anything. No light that it says it on. Any ideas? Its a 2001 no time to fill in sig right now