Name of part

I have a 96 Jimmy. When I put it in 4 wheel, the front wheels don't turn as they are supposed to. I do not know what the name of the part that is bad is. I was told it is a common problem and a pretty easy self fix. It is located near the transfer case if I remember correctly. I know this might not help, but if anyone knows the name of this part and maybe even a part numer, it would be greatly appreciated.

vacuum switch maybe?

located in the rh side bottom of the front axle housing is a switch and a cable. remove the cable and be sure it moves freely in its sleeve also check the diaphram on the other end (under the battery tray) I had the same problem

Transfer case motor? It mounts directly to the transfer case, and can cause 4wd not to engage. Usually an easy fix.