Your probably correct on the body causing lift after a certain speed. The theory of using no body to aleviate lift at those speeds would be spot on, but then you are faced with overcoming the drag. A front wing might be the key for getting the majority of the air to flow over the truck - I'd even consider one that extended beyond the wheels.
Losi 8T 1.0, Savage Flux - XL style, LST XXL, Muggy, 3.3 E-Revo Conversion and sitting outside 425hp, 831 Tq Dodge Ram Turbo Diesel. It SMOKES