Its definately the balance (and possibly the alignment) of the tires. Same thing happend to me when I first got my Contour. At about 65 mph, the car would shake. After the balance, smooth up to 100+ mph... oops... I didn't type that... it was for research.
My ride(Cardomain link) :
'95 Contour 16V Zetec DOHC,
Teal , ATX
Born on Sep. 4, 1995
86+ miles
*LEDs mods are on their way!*