I have a 95 Ford Probe that wont go into overdrive. What is wrong with it?

The car runs good except for the overdrive dont work. I drive it all over even on the free way and that makes the RPM's go up very high and I know thats bad for the car. So what might be wrong with the transmission making this happen? Also when it shift into 1st and 2nd it shifts very hard. By the way its an automatic.