I thought you had it working when you replaced the thermo switch. Did you replace it with a new one, or one from a junker?

Since the switch reads water temperature, try replacing the thermostat with a new one from the dealership. It's possible that the thermostat is sticking "open" and taking too long for the engine to come up to operation temperature sufficient to trigger the switch (176 degrees, if memory serves me correctly).

If that doesn't do it, I'd check the electrical connections on the O/D "on/off" switch, the wire from the temperature switch, the O/D OFF relay and the O/D OFF solenoid. I'd also check the O/D "on/off" switch for resistance to make certain the switch isn't degraded and can't pass the signal. The ignition switch should also be checked to make certain it has no internal resistance and the connections are clean and tight with no corrosion on the wires.

If you can find a spare ECU, it would be worth a temporary swap to see if the problem goes away, but I don't think that's the problem. It's more likely to be a bad sensor/switch/relay/connection.

I wouldn't give up and rebuild a transmission until all of the above has been done.

Good luck,
Frank