WELL! I was on my way home with the Dart. This is the first long drive since we built her and had her aligned. The build consisted of, swapping the drive line and rear from a 74 Dart. Changing our 1970 from a /6 manual drum car, 7 1/4 rear SBP to a V8, power disc, 8 1/4 BBP car. I used all the factory items from the donor car engine, trans, driveshaft and rear. The engine is a fresh rebuild, I just serviced the trans (new filter and fluid) and rear prior to installation. It has all new tires, and a new factory rubber front end part with a fresh alignment. I replaced both front caliber and brakes and rear brakes and 1 wheel cylinder on the passenger side. 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.