[RexNfx400]

How could it not be a trans problem??? lol I suggest you go to another/much better mechanic. Torque converter definatly has nothing to do with anything you desrcribe. Theres not any reason to even pull codes for this transmission. Except for the Converter.
I would say its probably like this...Differential and trasnfer shaft bearings have failed. This loads the transmission with very fine steel. This gets into the valvebody and govenor and hangs shift valves and the pressure regulators.
You need to find a better tech!