I have a 2000 Navigator with nearly 98,000 miles, many of them tough off-road miles. A couple of weeks ago, I noticed a slight vibration thru the steering wheel and drivers seat at freeway speeds. Took it to the local Ford dealer and they recommended new ball joints, front brakes and tires. Did all three but the vibration continues to get worse. Lately I have noticed a slight vibration even at low speeds and at 70+ it is like sitting in one of those massaging chairs. Took it back to the dealer and they could find "No problem at this time." Could they have missed something, loose or broken motor mount? Any ideas? Thanks.