Did you just purchase this vehicle or have some steering and/or suspension work performed on your vehicle? If the Wheels turn more in one direction as opposed to the other, it is a good possibility that the pitman arm is 1/4 turn off center on the steering gear box pitman shaft. With the wheels straight (not steering wheel as this may be off), put a piece of tape at 12 O'clock position. Turn the wheel all the way to the left stop counting the number of turns, repeat the procedure turning to the right stop counting the number of turns. The number between left and right should be within 1/4 turn. A simple alignment and centering the steering wheel may correct the condition. If the difference is 1/2 turn or more, close inspection of the steering linkages, tierods, etc, should be performed. As a last resort, the pitman are should be removed, the steering gearbox centered and the pitman arm reattached and a front end alignment performed. This should be performed by a qualified technician.