My wifes 2009 6.7L cummins started to vibrate the other day. It starts when you get up to speed and doesnt quit till under 5mph. But as you slow down it feels like your driving on washboard and then quits. We can hear it, feel it in the floorboard and in the steering wheel. Any idea's? The mechanics at my local dodge service dept are claiming the tires are causing it, but the truck doesnt have 38" super swampers on it. The tires are 33" nitto terra grapplers AT. I had the tires balanced, rotated, and aligned, with no joy on the problem. I really need help on this one.