I have a 2008 Chevrolet 2500 HD Duramax. When I try to put it in 4 wheel drive it gives me an error code on the dash says service 4 wheel drive. The indicator light on the selector **** says its in 4 wheel drive but the front axle never engages. Any ideas

 

need to check the codes, the position sensor in the shift motor likes to be a problem but without codes its just a guess