So are we talking like full Kelderman air ride here, or just one of Ram's air suspension variants or a normal pickup with airbags in back? If anything but the first option, it won't make a perceptible difference, other than air ride or air assist offer a couple neat ancillary features like
Note that higher distance from the body pan to pavement will allow more air to go underneath, which will create more air turbulence, which increases drag
Why track racers try to get next to no air passing underneath