Your diff is shot. The pin fell out and broke the extention housing most likely. Can be a cheap fix if the ring gear is not damaged. If its not you can just swap out the diff. If you dont think you can fix it get it TOWED to a repair shop. you have to seperate the ball joint and pull the axle out then take the cover off.
Here is one pic I will see if I can find another.
http://www.askbw.com/parts/parts/75410-10.htm