auto 4th gear not working...

85 4runner. when i accelerate up and when 3rd gear auto is at a point where i needed the overdrive on, after pressing the overdrive button the engine revs but doesnt pick up anymore speed. im assuming the 4th gear (overdrive) is not working. im confused. when the overdrive is shut off, the the trans gears grab again and move at a high RPM. whats the cause??