differential plug removal

[Peter Kirby]

Even with using a Hazet 17 mm allen key socket, I managed to round out the
socket head of my 90 Q's rear differential plug.  The metal of the plug is
very soft from corrosion and seems to be thoroughly bonded to the aluminum
casing of the diff.  Any suggestions as to how to either remove the plug
(I've tried soaking it in penetrating oils, including ATF, tried knocking it
counterclockwise with a cold chisel -- just rips it up)?

If I can't end up getting it out, what's the easiest way of checking/filling
with oil -- remove half-shaft flange, remove diff-locking mechanism?

Lastly, any idea how hot a normal diff should get after about an hour of
highway driving?  Mine gets sufficiently hot that I can touch it but can't
keep my hand on it.  Does this sound normal?

Thanks.

Peter