If I follow you there's a seal just inside the axle tube (almost flush with the outer end of the tube). If I'm correct, this seal can be replaced without the differential being disassembled. If you put a pry bar across the opening of the axle tube with one end of the pry bar just inside the seal (catching the inside lip of the seal) it can be pried out. Replacing it is just as easy, using a small hammer to push the seal into position.