Oh, good call, I forgot about the outer seals! I'm used to working on Semi rear diffs, so I might forget a few little things(no pun intended
). BTW, semi rears have no outer seals, they have gaskets where the axle bolts to the hub. It's been a year now since I've worked at reliable so my memory is a little fuzy....
Also, while you have it apart, you might want to check to make sure that the spider and axle gears are OK. If it's a posi unit, then you also might want to be sure that the clutch packs(most common) or cones(not so common) are good as well. If any of these go bad, it will generate the humming sound as well.