The rear tires leave a substantial footprint on the ground. Because no one really makes racing wings for a Freightliner, a wing from a Stearman bi-plane was repurposed.
The rear wing is a triple-element design to generate downforce as efficiently as possible.
What else is in the rear of the truck? A handful of heat exchangers to keep the various fluids cool are placed in available spaces. I can imagine the transmission fluid getting quite toasty. A Racepak system with a lot of channels is also mounted back there.
Mounted on the other side of the truck under the intake manifold are another pair of heat exchangers.