Oh, between the seal and cover. I see. He didn't say that but I probably should have deduced.

Those axle seals are usually made with VITON and may expand a bit when come in contact with transmission fluid.

One thing to check is the amount of play there is between the yoke and the housing. If the bushing is worn the yoke will wobble and create a leak. The output shafts are quite wobbly but the bushing is what keeps the yoke from wobbling to much.