your shaking could be one of three things.
1) wheel balance, the most likely one.
2) your front rotors are warped, (you only feel this around 55+mph usually)
when the steering wheel is shaking, (around 60mph) put your foot on the brakes. if the
shaking gets more violent when you hit the brakes, you have warped rotors. if it gets better, usually
tire balance.
3) the more uncommon issue is that your alignment is so out of wack that your tires are
fighting each other, but they would normally do this at low speeds as well.
-- i dont know how many times someone comes into work saying they are experiencing a vib. above
50mph, and i get to diagnose it... good luck.