Are you talking about surface rust on the housing or carrier or on the inside of either one of them?
I use spray Easy Off oven cleaner after rinsing any car part with cleaning solvent that has had oil or grease inside or on the outside of them. If the rust is inside the axle housing your om your own on that, sorry blush
If on the outside can you get it sand blasted? scope up

Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)