Make sure there are no dark patches when you wire brush and remove 'flakes'.
This would indicate the axle casing is porus and casting thin, - no amount of rust converter or paint will cure that. If it looks sound, make sure you thoroughly prep/degreaser before any coating you put on, or you are wasting your time.
Fingers crossed, at worst it could be a small weld job if casing thick enough, if not, others have changed the complete axle casing from Milners.
As Karl said, check that rear chassis, Stuzbot had a problem there.