Sounds similar to a problem a friend of mine had. He would get to within about 3-4 miles per hour of about 65, and then his whole jeep would shake. Once he sped past it, it was fine, and once he slowed down it was fine. Just in that little range.
Unfortunately, he never did anything to really fix it. So I'm all I'm saying is that you're not alone!
Sorry couldn't be more help!