The Stovebolt Site Forums The Shop Area Driveline Pictures of Steering Dampers, please

If you have a shimmy, its because some part(s) of your steering and/or suspension system or tires are faulty.

Installing a steering damper isn't going to make the problem go away.

These trucks aren't tractors. They track straight, don't wander, and actually handle pretty smoothly when all parts are functioning properly.

I'd start with the basics by swapping wheels front to back / back to front and see if there's a change. (inflate and balance tires, etc.)