Brake Booster Question

WELL! I was on my way home with the Dart. This is the first long drive since we built her and had her aligned. I was going to go to the brake shop today to have the booster tested.
The car drove great on the side streets, alignment was great and the car was running well.
Then I jump on the freeway and get up to 50-55 and everything is good for a while then I get this terrible vibration through the whole car. It shook as if I had a flat, and I actually thought I may be losing the driveshaft. I pulled off the freeway to the nearest parking lot and checked over the car. Everything seemed the way it should be. I convinced myself that it was the road and started on my way again. The car ran fine, steering fine everything seemed good. After driving for awhile it did it again. I pulled to the shoulder and waited and then continued on. I got off the freeway and took side streets home the rest of the way. The car seemed fine on the side street even after getting up to 45-50. It scared me to the point I'm considering throwing in the towel and selling it. This is my son's car and we built it together but I am really over all the trouble we have had with this. Plus I still have my power brake issue.
Why would this shake so badly. I checked all the wheel bearing, driveshaft and the tires are brand new. I think I'm losing my mind.

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