Harmonic pulsation / Vibration?????????

I'm eager to hear more about this. I have a vibration in my 64 Dart that only comes in when in motion at about 30 mph and 60 mph. all other speeds its smooth. It feels almost like the car is rolling over rumble strips on the highway. I can feel it in the steering wheel. Front wheels were recently balanced with new tires put on, driveshaft was professionally shortened and balanced with new U-joints installed about two years ago. This vibration has been there all along. I'm thinking I need to adjust my pinion angle as the car is raised a bit in the rear with air shocks for tire clearence.