2000 Mercury Marquis Front tire shakes.

Shakes or Wobbles problem
2000 Mercury Marquis Two Wheel Drive Automatic

I have a 2000 Mercury Grand Marquis and bought it with 140,000 miles on it. When driving on the roads at speeds over 50+mph the front end begins to shake. The shakes usually only dont last for more than a couple of seconds but I'm not sure what else to do. I've had the tires checked and balanced, but not rotated because the front and back are custom wheels with the back tires being a wider tire. I was thinking that maybe the front end might be the issue but wasn't sure exactly what and if that was the only problem. What would be a price range for a front end job and can you give me any insight to what may be causing the shaking?