We have an '06 F350 Dually and have no problem getting under and pulling our Everest. We have about 7" clearance between the fiver and the bed rails. Course, the fiver axles were flipped from the factory and the truck is not 4X4. The 4X4's I've parked next to sit about 6" higher than our truck, and I would think the only way one would be able to get under the fiver would be to flip the axles and lower the truck.
Larry