4x4 will engage
If it's anything like the explorer unit there's a little plastic bushing in the motor that keeps it from over centering off and not making an electrical connection. If you look around the vacuum T help section you can find one that's the right size to fit over the metal pin and replace the worn or broken stock piece.
Before you go too far, are you sure you are actually hearing the shift motor moving? Or are you hearing the relays in the shift control circuit clicking? In my experience, hearing the shift motor moving through the floor boards is nearly impossible. Unless you are under the truck listening while someone else pushes the button, its a nearly impossible feat.
May we assume that the 4HI indicator does not remain lit and therefore it is assumed that the 4WD is not engaged? Does the indicator light at all when the button is pressed? Have you observed the shift control circuit (behind the passenger side kick panel near the floor)? It has a red LED that should blink when the button is pressed and come on steady once the shift motor is locked into one of the 4WD positions.







