Ford Explorer 2006

I bought a 2006 Ford Explorer V8 after 10 years of driving a 1996 Explorer, so I expected similar performance. I use the Explorer for minor hauling, equipment, etc. on my farm, to drive to work and shop. I drive approximately 5 miles each way on gravel roads, with mud, snow, ice, etc. (depending on the time of year), before getting to paved road and interstate highway.
I have repeatedly returned to the dealership because the vehicle shakes and the tires appear to be out of balance at highway speeds every time it gets a little wet or snowy. One of these times, the vehicle shook so badly that I had to drive below 40 mph to get to the dealer (I almost called a tow truck). The dealer insists that mud is causing the problem and I must clean the wheels every time I drive by going to a car wash with a high pressure hose and placing the hose behind the wheel to clean it. Obviously, that is nearly impossible, especially when it is 20 degrees outside.

This is supposed to be a vehicle designed for this type of use. Does anyone else have similar problems? This is so bad that I am going to get rid of it as soon as I determine what other SUV with similar clearance is available without these problems. My family's contracting business is in the process now of switching from an all Ford fleet of trucks for the last 35 years to Nissan because of this problem and other issues with Ford quality.