+1Jeep Cherokees used a variant of that transmission. In fact the bellhousing and pump from the A343 gets used when people will put a Toyota / Lexus V8 into a Jeep. There's no aftermarket because these transmission are pretty much indestructible. Don't need to upgrade what doesn't fail.
I had the same issue with the drain plug on my Jeep XJ with the AW4 (Toyota A340). I ended up just buying a new pan, and replacing the pan was a PITA. The pan itself was fine, but disconnecting it from the dipstick tube was a royal pain. Finding a new pan was more difficult than I thought it would be, because like SwampSurvivor said, they are so robust there is very little aftermarket support besides external coolers.