I think its one of your front tires. Usually if the wheel is off balance, it will start shaking around 55-65 mph, and kinda get better or stop when you go faster. You said it starts off light then gets worse by going faster, so i think you might have a busted steel belt in the tire, or a small bubble in the tread. The guys balancing the tire should be able to see a small bubble in the tread when balancing..It could be just your balance is off, but its def one or the other...