probably worn out parts like highserria said, and you cant notice it untill you get it cranked with more stress and more air under the truck floating it around on the highway... i dunno... thats what i think
I had shimmy issues as well. I rotated my tires and that took care of the problem. This was back when i got my truck and the tires were awful up front. When i rotated them it shook really bad, switched em back and that fixed it. This was right after i cranked my keys too. Mine are cranked all the way up and i have no problems to date.
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