If if is the breather tube, just to the left of the diff housing on the left, looking from the back, it releaves pressure as things turn around in there. It is nessisary to keep water out of the housing. Normally the tube runs from the axel housing up to the body just about the axle. While it may not be an emergency, you should replace it as soon as possible. I would also change out the fluid if you don't know how long it has been missing, just in case there is water in there.