It seems since KIA farmed out or scavenged most of the parts (short of the body and easily manufactured peices) that went on the sportage, that it's possible that diff could be made by anyone!
I have Robs front and rear hardware now and have been trying to find time to get it in my shop and tear it apart for a closer look. I had too heard somewhere long ago that it was Toyota and that goofy van that toyota made in the late 80's and early 90's with the center mounted engine had a solid axle I think and may have been the same. It probably doesn't matter if it is Toyota as it was no doubt not popular enough to warrant any kind of aftermarket gears.
Everything has KIA stamped on it, but like the engine, we know KIA didn't manufacturer it, Mazda did, so what's to say they stuck with strictly Mazda for all their parts. Ford built their Escort, Tempo, and EXP with the help of 12 different countries and 9 manufacturers, so anything in the automotive world is possible.