[aerohead]

You might try an air dam first.Measure the ground clearance to the underside of the rear axle 'pumpkin' and create a front air dam as you see NASCAR teams using,which has the clearance of your pumpkin.
The air dam will direct air around and over the truck,away from the underside,effectively shielding all those components from the ravages of the 'wind.'
See what that does for your mpg.
Later,if you choose to spend some more time and money,consider a belly pan,which essentially covers all those naughty bits.
The forward 2/3rds of a pan performs almost as well as a full pan.
A full pan with diffuser offers the lowest drag.Nissan's LEAF has this.