those wheel arches seem hugely oversized. even lowered this vehicle will have quite some ground clearence, but if lowering is not an option perhaps adding some wheel skirts could be good. keeping it reasonable these could cover the the area abouve the side moldings, and these could be made form either plastic or aluminum sheet. a spray can of matching paint will makes them look as if they belonge there and they would give you some extra space to advertise your phone number or whatever
an undertray could also be a good thing, and appart from the aero gains (even if they would be small) it'll help keep the underside of the vehicle clean and keep dirt out of the engine... not hugely important, but an added advantage.
further what i'd do is look as a modest airdam extention, this might create a lower pressure below the vehicle, getting more airflow trough the grill thus allowing you to block it more without loosing any cooling.
Also i'd investigate if it's possible to install some larger dams in front of the wheels... again a shot spray paint, black in this case will render these mods almost invisible the anyone not paying attention... finally a little camm back spoiler at the rear will once again not stand out but might help airflow separation at the rear, also a strip that coveres the gap between the cab and the rear compartment.
just some ideas
