Could be a loose shock or worn shock. Is it a up and down problem or do you feel it as a steering wobble in the steering wheel? A good steering stabilizer will control a lot of wobble in the steering linkage. Since it goes away when you slow down, I would suspect the whole steering assembly. Have you adjusted the steering box to remove all play and slop?
Your bigger tires will compound the problem due to the extra weight and rotating mass. Recheck the shocks or replace them with extra heavy duty ones, then move on to the steering box and all the joints.
Joe
