Jack up the front of the passenger side of the truck. Grab the passenger side tire and rock it back and forth and watch the idler arm assy on that side. When the idler begins to wear, there will be excessive play in the steering which leads to the shimmy. I am assuming your truck has one, if not forget this post. My 1991 had this problem and the idler was the smoking gun. I purchased an aftermarket idler for about a hundred bucks. Good luck.