98 f150 shakes at 60mph and 67 mph

Have you checked your suspension? If you rotate the tires does it change, anything? As far as the engine, problems at higher rpms, check for blockage of some kind. The first two things I think of are fuel and exhaust. In the past, my vehicle, had vibration at one certain speed, above or below that speed, no vibration. Turns out I had a bad shock on the rear. good luck

Out of balance tires, bent wheel, worn or damaged suspension, out of round tires etc. A Road Force balance can solve difficult tire balance issues.