If you can hear the servo operate and the steering doesn't work, it's the servo. Just look at whats going on. The servo(plastic, black box with the wires coming out of it) is connected to the drag link(small shiny arm with the black ball joints on the ends), the drag link is connected to the bell crank(the inside part that the long shiny arms with the black ball joints that connects to the carriers, which your wheels/tires are attached to). Correct terminology is important. You have to know what the parts are called in order to properly diagnose a problem. Your manual should tell you the names of the parts. Now, before you replace the servo, pop the horn off(part that directly bolts to the servo with one screw) and see if the horn's splines are stripped. It's possible, but since your steering operates in one direction and not the other, it leads me to believe that the servo has stripped gear(s).