If your lucky, might just be a tire out of balance. You might have thown a wheel weight. The symptoms would be worse within a certain speed range, which seems to go along with your description. Also could be a shifted or bad tire belt. (It does happen.) If belt replace tire ASAP.

Former, now returned CEG'er! 95SE MTX, (AKA "The Road Rat"). Stock except for:TH fix, B.A.T. big brake kit, tranny cocktail and lots of re-insulated wiring! May yet be a 3.0! "Speed doesn't kill, stupidity does!"