rust & grease has the right idea, the spec is 1/16"-3/16". You'll need to check the adjustment of the steering box and the preload on the tie rod ends too. Any slop in either will cause the problem you are having. Your shop manual (you do have one, right?) will tell you how to adjust the box correctly, the tie rods adjust by turning the plug in until it bottoms out, then back off to the first cotter pin hole. Then follow r&gs procedure to set the toe. I set my truck with that procedure, it handles great.
