Curious as to why the "rust" is a concern? It's not like it's going to "flake off" and will be inconsequential once rear end fuid is added and it gets a "coat of
oil" on it.
Or, do you think the rust is deeper than just surface discoloration? Which in the picture is all it appears to be to me.
The exterior rust cannot be corrected short of paint and poses absolutly zero problems, now or LONG into the future. Rear ends rust on the outside if their not painted, thats what they do! On many metals (such as rear end housings) they will take on a surface rust and it will act like a coating. It's not like the rust is going to "eat through" the housing or something.
Ernie