Wife was complaining that the Jeep stalls while backing out of garage. It also stalled doing the same thing at a store while the vehicle was warm and not sitting more then 15 minutes. So I gave it a try myself this morning. Started fine, put it in R and it engaged instantaneously thanks to my check valve mod, backed out about 5 ' and it made a horrible crunch and stalled the engine in its tracks.

So what do you think torque converter or something more?

The only time I've heard a crunch type noise followed by an immediate engine stall is the couple of times I tried to pump up a drained TC by bringing up the RPMs while in gear. That produces a hard gear engagement and then you can drive 50-100' before the noise and the engine stalls but that does throw a code. Not to sound negative about your check valve mod but I wonder if the valve is sticking, I've seen similar valves do that, and interfering with the movement of ATF through the system?

About a year ago I thought my TC was out, when I would put it in drive or reverse it would stall. Took it to a few transmission shops that over the phone swore their scanner was the best and could communicate with the TCU to find the issue. About 4 shops later not one if them could connect to the CRD. I buddy that used to work for a shop near by said that the owner was a good driveability guy and has good computer equipment so I brought it to him. He found it was a wiring harness behind the alternator was rubbed into and when it was shifted into drive or reverse it would move the engine a little and contact the open wires.Took him some time and searching around but he found it! He is good and he has seen the jeep a few more times since then for some other wiring ABS stuff.

So you could check that out, I guess it has happened to a few others on this forum but I don't remember a "horrible Crunch".
Good luck.