If the leak is
from the axle seals, it's probably too late and you're looking
at replacing all the seals - which would mean you could
continue using synthetic. If the leak is at the plug, maybe
there's no problem (until there is, in which case it's too
late and you're looking at new seals).
So if the seals
aren't leaking yet a flush is a good idea. If it was me I
wouldn't assume ATF has no synthetic additives or other odd
stuff. I'd use a light weight non-synthetic motor oil and
jack up the rear wheels while you spin it up to splash the oil
around. (Motor oil doesn't have the high pressure additives
a loaded transmission needs.)
Motor oil
is built from the same stuff as hypoid lube, so you know
they'll mix well and give a good flush. ATF has a lot going
on in there, and it's anyone's guess how it'll mix with gear
lube.
Chris