A sealed bearing, seals contaminants out of the bearing that is greased and sealed. It doesn't seal the lube from seeping through the bearing. You do need to use the factory axle seal to seal the diff. The factory uses greased tapered bearings. They are not oiled buy the diff. Sealed green bearings do not need service , they are sealed with grease. But you can take the seals off clean and re-grease them if you need to. Those seals to not hold the lube.
Your problem is at the inner seal. Housing or axle is not sealing the diff.
Axle bearings take grease not gear lube.
Some trucks and trailers have wet hubs cars do not.