1999 Ford Windstar overdrive light comes on at highway spee

The overdrive light comes on when driving at highway speeds and I have to pull over to the side of the road and turn off the engine and restart the engine to get the light off. Yesterday, I drove about 20 miles on the highway with the overdrive light on and when I stopped for gas about a quart of transmission fluid came out onto the driveway. The fluid seemed very hot. The van seems to shift just fine, but what could be wrong?