I have also stopped a shimmy by loosening the large lock nut on the steering rack and taking the slack out of the tension adjuster.
If you loosen that large lock nut, then have someone turn your steering wheel back and forth very slightly (you can feel the play, or slop in the wheel) while you are turning that big adjuster with the flathead slot in it, just enough to take the slack/slop out of the wheel, you will have a much tighter feel to your steering, and prolly no longer have play in the rack which allows your front wheels to shimmy back and forth, ever so slightly....