The design of the axle doesn't allow all the lube oil to drain from a rear wheel bearing. There is a pocket that holds the lube around the bearing, and the axle extends past the bearing, which is on the outside of the axle.
As far as capacity, the 3/4 ton differential holds less than one gallon of gear lube. I can't remember the exact amount, somewhere around 3 quarts. If your lube is black, it could use changing.
The wheel locking problem sounds a brake sticking, either from a broken return spring or adjusted too tight.
