Please note that I'm not recommending this technique for others, though it
did work for me in a similar situation back in 1972.  I was on hardpacked
snow in Flagstaff, AZ on a steep little downgrade.  The road had quite a bit
of crown and ended on a major street with a fair amount of traffic.

I was a stupid kid and didn't really have any idea what I had gotten myself
into, as the brand new rental van started to accelerate down the hill.  If I
even so much as touched the brakes, the van started twoard the drop-off on
the right side of the road.  When I geared down, the rear end started for
that same road edge.

Youth is the mother of invention.  I briefly stepped on the brakes and
locked up the wheels.  While the wheels weren't turning, I slipped it into
reverse and then gave it gas while letting out the clutch.  It worked!
Pretty squirrely and snow all over the place, but I was able to steer and
the rear end actually kinda stayed where it belonged.  Were I in a similar
situation, I wouldn't hesitate to do it again, though I sure hope I'm not in
that position again.

Karl Wolz