When I first got my 97 Jimmy, 4wd did work. I put it in 4wd and the tires did the usual hard to turn catching that 4wd tires do on pavement while in 4wd.
I put on a 2" lift with 31" mud tires and went to go test it across the street from my house. I was very close to getting stuck and figured something was wrong becaus eI went in the same spot with my stock tires and didnt have a problem. My brother (who was across the street) told me that only my back tires were spinning in the mud. I then jacked up all 4 wheels and, put it in 4hi, and only the back tires spin.
Whenever I push the 4hi button on the dash, I hear an electric noise like its supposed to sound but 4wd doesnt work. I figure its something in the transfercase. What could it be?

 

search the site, your not the first to have this trouble. check the do it yourself section too. look for a thread called 4x4 no worky and there should be another too. Still got questions after that, post them up in the proper forum.