Cragar SS's? Are they the Uni-mount type?...the ones with the elongated holes?
If these type aren't mounted just right, they will vibrate...but at higher speed.
Could it be a torque converter issue? I had problems with a Ford Crown Vic. The converter went bad. Between 50-55MPH, it felt like you were running over a cattle grate.
I'm a hobbyist. Not a professional. Don't be hatin'!