transfer problem in 97
 

I have a 97 GMC daily driver and when I put it into any 4x4 it grinds and goes nowhere.It has a floor shift not a push button.Now if I crawl under my truck and unplug the sensors on the front pumkin it quits.So in other words when I go have fun in the mud I have to plug it in.i have replaced the module?on the front axle and still does it.Could I have a gear hung in the transfer case?Are they a pain in the butt to work on?Any help would be great.Shes finally paid for so now I can fix the damage I have caused her over the years.
Thanks
Corey