Yep, you can adjust them on the truck without problem!
To make it easier, i usually put a floor jack under the center of the truck and jack it up to take the weight off. You will have to figure out how to get the spanner wrench on so that each turn is as efficent as possible. Just undo the allen head set screw and turn.
I would say that every 3 turns will yield you .25" of lift.