1970 Dodge Dart with vibration under load

I have a vibration problem with my 1970 Dodge Dart when it is under load. There is no vibration when I am driving and put it in neutral. It has a new rebuilt motor and transmission. The steering is all new as well as the new brakes and bearings. I have had the car aligned and tires balanced but still have the vibration. Does anyone have any advice or ideas what this could be?