If it is an 02 or 03 there is a problem with the auto lock. There is an air actuator that will fail over time and when not used often. All new Fords have it unless there is a stripped model without. then it would have to have a floor shifter and you said you flipped a button. The whole idea is that if you know you are going into crap you can lock the hubs, the auto will not engage until it senses a slip and by then you are into it and stuck. The auto works well but the locks make it sure you are in 4 wheel drive. Plus the autos tend to disengage in reverse for a second, just enough to bang back in.