[Blue Bowtie]

The transmission diagnosis was relatively easy because you gave a reasonably thorough description of the symptoms. The 4L60 is tough on the stock sun shell, its hub, and the 3-4 clutch. Hopefully, the repair shop used a quality replacement parts and not a stock shell/hub.

The overcooling is not likely due directly to the repaired transmission, but possibly something that was done as part of the repairs. Regardless, there is obviously a problem, so it needs to be diagnosed and corrected.

The first thing to do is confirm that the cooling system is completely filled and has no entrapped air. Air pockets can cause the temperature sensor to report a low temperature, then rapidly increase as steam flashes up to the heads and sensor.

The second thing to do is to verify that coolant is flowing while the engine is running. If there is no coolant flowing, a similar situation can occur.

The next thing to do would be to check that NO coolant is flowing to the radiator while the engine is warming up, and the thermostat is supposed to be closed. You can remove the radiator cap with the engine cold, start the engine and observe the coolant level as the engine warms. It should increase slowly, then rapidly drop once the thermostat opens and coolant flows to and through the radiator.

Of course, it would be good to verify that the temperature gauge is correct, and scan the PCM for the DTC(s) causing the MIL to light.