Toyota Repair: toyota transmission, overdrive switch, engine vacuum

Question
Thanks, Mr. Ritter.   My 91 Previa van runs properly in the first three gears forward.  No apparent slipping, and shifting seems smooth.  If I hold my foot on the gas it will stay in gear, but shifts up when I release it.  it will not go into fourth gear, as if I had the "overdrive" switch off.   On occasion, reverse will not work, but it will after the car warms up. this isn't very often, maybe once in 2-3 weeks.  I believe there is a tube somewhere that connects the engine vacuum to the transmission.  Maybe that is causing the problem.  I've been told I need to replace the transmission, but if so why no slippage?

Answer
Toyota does not use vacuum modulators on their transmissions, what you were told is probably correct, if the transmission fluid is at the correct level it's an internal transmission malfunction, transmission parts are very expensive so trying to repair it makes no sense, I would recommend using either a used transmission or a remanufactured one from an independednt rebuilder.