Shaking/Shuddering while towing trailer

If your tow vehicle is porpoising (rocking front to rear axle like a fish jumping out & into the water), likely there is not enough weight on the front axle meaning your bars are not pulling up enough. It should take some effort to lift the bars into place. If you get to the level-with-frame position too easily you may need to tilt the ball assembly rearward so that the bar tips are nearer the ground. Then you are loading the spring bars more to pull them to level position.