That tells you that it isn't because of an electrical problem in the TCC solenoid. The TC clutch may still be stuck on. Crud in the valve body or elsewhere in the transmission is preventing the TCC lockup hydraulics from losing pressure and disengaging.
Here's a test, just be careful and don't get anyone hurt. Pick up the drive wheels, start the engine, and drop it into gear. With the wheels off the ground, there won't be any resistance and the wheels should start to spin. If the engine can spin the wheels in the air, but stalls when you push the brakes or otherwise force the wheels to stop, then it is likely a problem with the TC clutch. You'll probably have to take at least the valve body out so you can clean it up real good.