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
Check the axles they may have worn cv joints. See if you turn the steering slightly one way or the other and if vibration lets up. If it does then it's probably one of the cv joints. You can also change tires from front to back and see if that makes a difference. If it does it's a tire problem. Also have your lower front bushings checked out for wear.