Tire vibration, wobble at 45+ speeds.

I own a 2004 Cadillac Deville and at 45 mph I being to experience a vibration in the wheel, I feel it in the steering wheel and in the gas pedal. The problem gets worst as speed increases and I notice that other cars move away or back off away from me. I have changed my tire rods, rotors, my air shocks, and recently within 1 year replaced my tires. I have taken it to the dealership and they continue to tell me that everything is tight, but I still experience the same problem