1996 Windstar Code P1742: Torque Converter Stuck Issue - Engine Stalls in Drive/Reverse

My 1996 Windstar is experiencing a persistent transmission issue indicated by Diagnostic Trouble Code P1742, which points to a suspected torque converter problem. The vehicle starts smoothly and runs well at idle, but as soon as I shift into drive, reverse, or second gear, the engine stalls immediately. In low gear, everything operates normally—no stalling occurs. Once the van is moving, it drives fine until I begin slowing down for a stop. At that point, the engine again attempts to stall, and I must remain in low gear to avoid stalling. I have already replaced the torque converter clutch assembly (TCC) and tested by disconnecting the TCC ground wire—both actions did not resolve the issue. Transmission fluid is old but not burnt, and the level is within normal range. The vehicle has high mileage and shifts smoothly under normal driving conditions. However, it fails consistently when decelerating or shifting out of motion. I'm wondering if a lock-up piston could be stuck in place—why would this only affect higher gears? I've found no online references to this exact symptom on a 1996 Windstar with P1740/P1742 code. Additionally, the vehicle does not allow reverse operation due to immediate engine stall upon engagement. Could there be an internal valve in the transmission valve body that is stuck and preventing proper fluid flow or torque converter lock-up? I'm at a loss as to what's causing this behavior despite normal shifting under motion.