73 Dodge Dart Sport rear end shimmy

Shortly after mounting new Cragar ss's after cruising at 55 for a few miles, you can start to feel a slight shake that slowly and progressively get worse to the point you have to pull over and stop, then it goes away until you cruise at 55 for a few miles then...repeat. The wheels and tires are new and balanced, front end recently aligned, just added a set of shims to the leaf springs to level out the drive shaft. It has air shocks. Very frustrating, only started doing this within 2 hundred miles as we only drive too and from regional/local shows. Any Ideas? where do I start?
Thanks in advance.