1973 Dodge Dart sport speed related shimmy

Hey everyone, a follow up to a post I had in regards to a '73 Dart Sport shimmy. It starts after a sustained speed 45 up for around 3 miles, then you can slowly feel it shimmy and within a half mile you have to pull over due to shaking so much, It feels like it may be coming from the right side but it isn't steering related directly, however if you let it go you will feel it in the steering but nor more than you can feel it in the entire car/ it is a 318 w/a tf727. I've shimmed the pinion angle, all wheels tires, drive shaft drums have been balanced. Rubber shackle cushions are good. I can put it into neutral while it's shaking and it doesn't change, when I apply the brakes you can feel it in the pedal and the shaking slows when the car slows. The only way to get it to stop is to brake and come to a complete stop, then start going again for about another three miles then the whole thing repeats. New wheels and tires did not change anything either. All wheel bearings is fine and both u-joints have been freshly replaced. Any Ideas? Thanks in advance. It all just slowly happening out of the blue