2006 Chevy Cobalt Braking Causes Steering Wheel to Shake

Recently had a new set of tires put on at a reputable national chain and noticed two weeks later that while braking at highway speed (70mph), the steering wheel shook a bit at the bottom. As I went along further and braked, the shaking became worse and worse until it felt like the front end of the car was shuddering. Took the car to the dealer, who checked out the front rotors, calipers, brake pads/shoes, steering wheel itself and the tires. They said they were stumped, but were going to check to see if the engine was shifting inside during braking. Called the place where the tires were put on and asked if the tires could be causing the problem. They said it was possible that they weren't balanced correctly and to bring the car in. Could it be the tires or does it seem like something else?