The back hub seals don't generally fail. If theres a rumbling noise it would indicate that your swivel bearings have collapsed - this allows the seal to move well away from the swivel and the grease to leak out. Replacing the swivel bearings is at least a three hour job if your equipped to do it (it's easier to take the swivel off of the axle).
Now, quick check. Jack up the leaky corner and give the wheel a good shake. Grab the wheel top and bottom and shake. Then grab it at either side and shake. Movement in both top/bottom and side to side indicates a wheel bearing, major movement only top and bottom says its swivel bearings. Major movement side to side indicates a steering joint. That'll be the big teller.
Your problem now will be getting the parts before you need the vehicle - what with it being christmas and all.
Hope that helps.