1999 GMC JImmy 4×4

The 4×4 switches on the dashboard lite up when changing from 2×4 to 4×4 and seem to be working normally but the vehicle is not engaging into 4×4. I have had no previous problems with the 4×4 at all and only had some engine repairs done in the last year. Is there something under the hood that could have been disturbed or disconnected to cause the 4×4 not to function any longer