Is the axle assembly properly vented? If not, after the oil heats up the pressure will push it past the seal. My father just went through this. Not saying this is exactly your issue but it is free to check and an axle vent is super cheap.

Ryan "Bigs" '73 Duster (BLKDUST) - Black, 100% factory sheet metal, flat hood, 346 cid, J Heads, and a bench seat. http://s268.photobucket.com/albums/jj1/bigsbigelow/ Best to date: 12.40 @ 109 mph