Welcome to Ecomodder!
I found that your coefficient of drag is 0.44, which high but about right for a pickup truck.
I'm going to post a picture of a VW pickup to get an idea of what you're working with. vvvv
An air dam will help divert air from going under the truck and somewhat block the tires off from the airflow, at the expense of increased frontal area. The belly pan, while more work, provides greater gains IMO. It smooths out bottom of the vehicle, which is usually very aero-dirty. Built right, it can prevent air from collecting underneath the rear bumper and cancel its "parachute" effect. Using both in conjunction would do nothing but help
On my Escort ZX2, I noticed a big difference in coasting and the amount of throttle needed at speed when I did a front and rear pan. You might can garner some ideas from my Aero thread in my signature. I have reduced aero drag by 18.5% with my mods.
I do not have any information on an aerocap specifically for a Rabbit truck. I suggest checking out some of the members with Toyota trucks in the Aerodynamics section. There is a lot of ideas and improvising in their threads.