Front Suspension problems of the 2006 Mercury Mariner

I purchase a 2006 Mercury Mariner on February 18, 2006, at price Lincoln Mercury in centennial, CO. When I purchase this vehicle it had 15 miles on the odometer. I was instructed by the sales rep that I should avoid driving this vehicle over 55 mph to allow the engine to break in properly. I starting having a shimming and vibration when the vehicle get to 60 and above. This shimming is coming from the front suspension. I have taking this vehicle to two different Mercury dealership trying to get this problem resolved. The last result by one of the dealership was to replace all four tires, they replace all for tires and I am having the same problem. I contacted the Ford motor CO to report the problem and possible get them to replace or repurchase the vehicle which they refuse to do so. There response to me is that this is something I will have to deal with, because this is the normal behavior of this vehicle. I am writing to you for help because it is hard for me to accept that this vehicle operating the way is as describe by Ford. This vehicle drive smooth until it get to 60, and I have taken is back to the dealers five or more time for repairs for the same issues. Please help in resolving this problem.