I see your truck is a '10, so I'm guessing you've been through a previous winter with it. Sometimes cold tires give a symptom that can be somewhat similar to a shimmy. Are you sure you're not having a cold tire feedback instead of a shimmy? Also some tires as they age can get cold feedback until they warm up a bit. Do you get this feedback all the time, even after quite a bit of highway driving? Did this just start up this winter?