I have this exact same problem in my '95 Contour GL. On the freeway between 65-70 the front end and steering wheel begin to shake. Goes away under 65.
I thought either tire, wheel, or maybe ball joints. Haven't done anything about it yet though. When I had it jacked up the other day I checked the wheel for play, and while it had a little bit it didn't seem totally shaky or sloppy so maybe ball joints are okay.