There is a vent on the top of the differential housing (the pumpkin). When the air inside the pumpkin gets hot, the pressure lifts the vent cap, allowing pressure to escape. Problem is that the becomes plugged with crap and doesn't work. The pressure then forces fluid out the axle seals and onto the brake linings and drums.

To fix it, unscrew the vent (it's maybe 1/2 inch in diameter and 1 1/4 inches long) with a open end wrench and clean it out.