I put a 2" body lift with 31's on my 97 Jimmy. Went to test out 4wd and it wont switch into it. Ive had this Jimmy for about 3 weeks and when I first got it, 4wd worked fine.
When I press the 4hi button, it makes a shrill/electric motor noise, but the front axle does not lock. When I press the 4lo and switch into drive, it gives the usual loud clunk, but the front still doesnt spin.
What does this sound like? What should I check first? Please be detailed in how to check what your talking about.

 

First things first(and cheapest) check all your vacuum lines and replace as needed. There are three lines coming from the a switch on the drivers side of the transfer case, one of them is a vent, one of them runs to underneath your battery where the 4wd actuator is found(another possible culprit) and the other line gets vacuum for the switch. Check them all for cuts, and check the 4wd actuator for cuts as well. I had the same problem a couple weeks ago and it was the line running to the actuator, replaced where the line was cut and it still wasn't working so I checked it out again and it was cut in a second place behind the distributor. I replaced another section and now it works perfectly.