If your wheels cannot move - then you either glued to road , or your wheels pinched by fallen something or you got traction to road, in all other cases your wheels spinning, but truck couldnt get moving... Engaging of 4wd isnt done by measuring your position with GPS signals while truck moving, but by spinning of gears 8) So if you stuck like that - you still must be able to engage 4wd by just spinning wheels for a bit of time.