Pete,

Are you talking about RUST as in iron oxide as in the ugly brown stuff? If
that's what you are seeing, then it's from the body and not the transmission
housing. The tranny resides in an aluminum alloy case. You'd be getting white
curd if the tranny housing is oxidizing. I would just wire brush the tranny case
and paint it with a good quality paint. However, if what you see is a reddish
brown, I would look above the transmission for traces of rust under the body
of the van itself.

BenT
Vanagon Cafe