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If the solenoid for sure is good on fresh plates in the transmission, I would check the valve body as you suspect. There may be a gouge in it, a piece of gasket leaking by one of the valleys, or something else that is causing the pressure to drop away for that particular gear. With you getting the other gears, I doubt it is a clutch issue in the transmission from the disk flipped for you would lose all forward gears. This sounds more like a pressure issue with either a defective solenoid by having debris in it (even if it ohms good, could still have something blocking the port) or a valve body concern.
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