1991 Ford F 150 91 F 150 M Thinking Overdrive Out On Just Revs Until Let Off

I have a 91 Ford f 150 I'm thinking the overdrive is out. When overdrive is on it just revs up until I let off the gas then it jerks really hard into gear then goes out again but as soon as I shut the switch off (on the dash) the transmission shifts and works fine. But seems how I use my truck on the highway quite a bit I would like some answers and prices to see if I could get it fixed for fairly cheap