1997 Toyota Camry overdrive doesn't work

The over drive seems to have gone out in my Camry. The car will drive fine if the overdrive is off, but if I turn it on it bogs the car down. It almost feels as though it has down shifted, but the rpm stay the same. Is there possibly some module or something that could cause this. Or is it more than likely junked?
Doing some reading it sounds like it could be a speed sensor. The "Transmission failed" at a repair shop that I had to take it back to 3 times for the same unrelated error and there where other random not right things with the car. Would it just be the overdrive if that where the case, and how would I go about checking that