A long-shot.

You could try pushing/pulling the car with the engine idling in drive, see if the rear pump will generate enough fluid flow to break the piston free.

Also, if it's cold, warming it up might be all that is necessary. This might be due to shrunken seals allowing the fluid pressure to bypass seals and not operate the transmission.

Either way, start with the water pump and then warm up the engine. Wouldn't hurt to buy a magnetic oil-pan heater to warm the transmission fluid first if it's outside in the Ontario winter.

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Gone to the dark side

- Jeff