slightly vibrating going around 50mph

What kind of frequency does it vibrate at? Is it more consistent with tire speed or is it alot higher, more consistent with driveshaft speed? And it stops when you go above or below 50mph?
 
i have the same exact thing from 50-55 the steering wheel shakes but at no other speed does it do it -- it all started right after i was driving on the highway and two cars in front of me slammed on their brakes so i slammed on my brakes REALLY hard to avoid hitting them