I have a 2012 F 350 crew cab eight foot bed with FX4 package. The truck has a shudder when you accelerate from 0 to about 5 mph when I'm towing my 8,000 pound 5th wheel. Once you hit 5 to 10 mph it goes away and everything is smooth. When the truck is empty there's no problem either. I took it to the dealer and they said we have to shim the rear suspension. Does anyone have any suggestions or fixes ?