It might be that the rotors you bought weren't balanced. They normally are from the factory but it wouln't be the first time I've seen this. Same for the shaft. If it is occurring @ less than highway speeds it's more than likely a separated belt in your tire. Get the fronts re-balanced and look for lateral runout of the tread (tread moving side to side), or jack up the car and spin the wheel. If the tire has no tread in one spot or a lump in it-it's separated.