THAT!If you get snow in your rims it will sound like ass and vibrant like hell. I would put money on that. Do you have steel rims or alloys? This typically happens worse to steelies as they are closed off more, but can happen to alloys as well.
i took my last car to the dealership last winter because of the shimmying at highway speeds...turned out i had packed snow in my rims, and that little bit of weight was throwing the balance into dismay.
went to the carwash, done deal. it was allllll smooth again.