Play in wheel bearings and swivels can contribute to steering wobble, too. Jack up the front wheels and check for excess play at the wheel edge. The steering box is your next target, get someone to hold the output drop arm and see if the steering wheel has play in it. You can adjust some of the play out (or at least even it out across the steering arc). An indication that the worm in the box is worn is more play in the centre of the steering than at the extremes.