Identify the cause, then replace defective parts, tighten the loose bolts, or fix the alignment. You didn't bother to mention at what speeds this occurs, what leads up to it, where you feel whatever it is you feel, etc, so I can only offer a few suggestions. If it occurs at highway speeds after hitting a bump, check if the oil has leaked out of the steering damper. If it started occurring after an alignment, "caster" may have been adjusted too high. If it has a lift kit and over-size tires, learn to live with it or put it back to the way it was designed. If you feel a wobble in the seat or steering wheel at low speeds, suspect a broken belt in a tire. If you only feel it when braking or there's a pulsation in the brake pedal, suspect warped brake rotors.