Success!!!! After it a little heat and the impact driver, I was
finally able to remove the plug. Even with all of that, it was still
tough getting it out! The impact driver did a number on the slot, to
the point where it was leaving a "bow tie" shape in the plug. After it
turn about 1/2 a thread, I was able to get it out with a ratchet with a
screw driver attachment.

Unfortunately, I found what little gear oil that was left was tan, so
there most have been some water in with the oil.