Spray the inside with PB Blaster. Let it loosen up the rust and wipe it off. Do it several times then pour some oil in on the bearings. See if it rotates smoothly. If so run it. If not get a bearing kit from dr diff and replace the bearing.
Agree with the post above, get what ever rust that will wipe off, check that the hub rotates smoothly and don’t worry about it. The lube oil in the diff will prevent any additional rust.
