FOLLOW UP: Just got my XJ back from the transmission shop. It was in there for over 2 months, during which he completely rebuilt the transmission and the torque converter, replaced the valve body and the transmission control module and even the transmission case, and at the end he said that he and all of his fellow transmission shop owners on a network were all completely stumped as to why none of that fixed the problem. He did not charge me for any of this, because he said that he does not charge if he can't fix the problem. He was still pretty much 100% sure that the transmission would be the problem, but at that point he said the only thing that I could do would be to actually replace the entire transmission with a rebuilt one, and hope that doing that would solve the problem.
So... It seems that one of these things has to be true:
1.) All of these Transmission Specialists plus a certified Jeep Master Mechanic are all completely wrong, and the problem is not related to the transmission at all, and my transmission mechanic invested over 2 months of time plus the parts and labor required to rebuild the transmission and torque converter etc. based upon a wrong assumption that the transmission was the problem.
OR
2.) Somehow my transmission has such a rare and freakish problem that it was able to avoid being fixed despite the transmission almost being completely replaced.
I'll just have to keep searching the internet to try to find someone else who has experienced this obviously very rare problem.