Mudflaps won't help. It's a simple fact that, due to the flat back design of SUVs, turbulence is created that throws dirt up and all over the back of the truck (and therefore all over your bikes). That's why five minutes after washing the truck, the rear window and hatch is covered with dust. Unfortunately, this type of airflow is a fact of life with an SUV, thus the reason why a windshield wiper is always installed on this type of vehicle.