I bought a 1992 Jeep Cherokee recently and I'm not sure if overdrive works right on it. How can I tell?

I don't know if there is a switch to turn it off or on. If not, how do you engage overdrive? Is there a light on the dash or somewhere that comes on when it is in overdrive? I've been having trouble with the front end of the Jeep shaking when I drive on the interstate between 45 and 65 mph. I understand what overdrive does, but I have never owned a vehicle with it before. I just don't know if it is working right on my 1992 Jeep Cherokee.