ok a few things: A its extremely hard to lower a 4X4 because of the front axel.
you can only lower it so much before it becomes useless.
since you have leafs all around you MAY be able to get buy by flipping the axel on top the springs(flip kit) BUT once again whole host of front end problems.
the highway milleage sucks because its a truck lol that and a 5822 lb box on stilts isnt ideal for aero...
what people do on 4x4 s-10's and regular s-10's is have an air dam of some sort, deflect it around the front. the 2wd when its lowered it seems to boost mpg a little (thats extreme lower tho, like 5/6 (5 inch drop in the front 6 in the rear) and when said truck is that low you have about a half a soda can ride height. thy sometimes get hung up on speedbumps. the 4x4 u can only lower 2 inches as the driveshafts start issues.
dunno how much thats gonna help on a fullsize truck...(my guess is nothing)
now airbags are yes for raiseing and lowering the truck. BUT unreliable and costly, meanign not a good idea for a daily driven vehicle.
there seems to be come confusion between air shocks and air ride. air ride the airbags replace the springs (the ONLY things holding up a car) hence less air lower it goes till it hits the ground.
air shocks are merely adjustable shocks and really dont raise or lower the vehicle much (inch or less) much like normal shocks, thier there to dampen the oscilation of the springs. (bad shocks car keeps bouncing) thier meant so you can carry heavier loads and safely carry them and adjust as necessary.
personally i think your up a creek. a bed cover may help some. if not then it makes a VERY nice storage area. im so glad i made my own.