[MoparNorm]

I can't tell what is wrong, but I can tell you that you should never, never, ever tow in overdrive.
That's why there is the overdrive off button.
I'd suspect that the under load issue is related and in the engine management system. Maybe something in the cam/timing chain/CPS?
ImperialCrown will likely have a better answer.

 

[MoparNorm]

Welcome to Allpar. I would pay attention to the cat fault code. It is most likely real. The reason that it takes a long time to set is that the PCM wants to be extra, extra certain that the problem is really the cat.
If you 'thump' the cat, does it rattle inside?
If the internal ceramic element has broken up, it can intermittently block the exhaust exit and cause exactly what you describe.
CARB no longer allows 'direct fit' cats from what I understand.

That is correct, the CAT must have the correct part number or an CARB. Exempt, Executive Order number on it.

I was remiss, welcome to Allpar, I see your location is close to my old stomping grounds, not much in the way of shops up there, but a trip to Bakersfield might save you some money in the long run, if you're going to need a CAT.