Paul,
When you take out the 4 nuts and pull the axle, some fluid is going to come out. If is the smelly standard gear lube, then just put that back in, but if it does not smell real strong, it is probably
synthetic gear lube like
Amsoil. I would fill it back up with that if that is what it has in it. If it is a posi unit, dont forget to put some additive in for the posi unit, depending on how much has leaked out or comes out with the seal change.