1988 Toyota R22e Pickup carrying a Wiinebago

The overdrive on a 1988 Toyota R22e pickup carrying a Wiinebago does not work above 4000 feet altitude, according to the owners manual and my experience. How can I modify the pickup automatic transmission overdrive so that it works above 4000 feet altitude?

On long flat roads above 4000 feet, I have to keep speed 40-50 to not race the engine. Here the engine has plenty of power to run 60 mph without taxing either the engine or the transmission.