97 explorer awd (not working???)

hey im new to the fourm, but we in michigan just got 5" of snow, my wife and i just baught this eddie baurer explorer with the 5.0L so its awd, but i got in the car and i was backing out allright. but something just didn't seem right so i asked my buddy to watch the front tires and see if they spin, so i gave it some gas and just the rear tires spin, how do you tell if the AWD is working? i know its Diffrent than 4x4 but i also know that the rear tires are not supost to sit and spin, any ideas if what it might be and how much it might cost to get fixed.
thanks for the help