4x4 Not Working - 2002 Gmc 3500 Duramax

[Big Tom]

Like the topic says, we have a 2002 Duramax at the farm and the old man thinks he left the 4x4 engaged during highway travel and now when we try to push the 4x4 button and it lights up that it's in 4x4 but it's not working.

 

Haven't tried yet to figure it out but thought we'd come ask here first what maybe the issue or what can we test/inspect without to much trouble. Never really troubleshot a 4x4 system before so even some basic test procedures would be helpful