start by checking your alignment, suspension, and tires. i know that on some imperfect highways my csvt can do a little shimmy. at 80 i very much doubt that a lack of spoiler is causing your shimy. soon though others will post and either correct or confirm this

00 black/tan svt, #2052 of 2150, born 2/1/00 formerly known as my csvt "Nothing in the world is more dangerous than a sincere ignorance and conscientious stupidity." -Martin Luther King, Jr.