I am not an Isuzu expert, but I wanted to give you some things to check;
Have the mechanic check if the transmission control unit is keeping the converter lock engaged for some reason. It would be an electrical problem within the separate control unit, and that would explain why replacing the entire transmission doesn't help. They would all fail because the root problem is in the control unit that is located in the car somewhere. Your mechanic could try to obtain a wiring schematic for the vehicle to identify the wire that controls the converter lock (TCC) and then backprobe it to monitor the signal on a test drive. It should release at least when the brake pedal is applied, if it didn't, your car would shake and stall every time you stopped.
Also, if the control unit is trying to engage two gears at the same time, it would cause unporedictable problems. Maybe try a used control unit from a wrecking yard before spending money on a new one? A good manual would tell how to troubleshoot and test your transmission model. It would involve measuring fluid pressures via a test port too.
I would talk to a shop that specializes in transmission repair.
On some vehicles you can unplug the control unit and drive the car as if it were a manual shift. It would disable the converter lock too. Not sure if this can be done with your vehicle, it could help isolate the problem. Just need to be careful with the test drive and catch any problems early, before more damage occurs.
That's all I have to offer, I hope other experts would chime in and help you further.