The 5.9 is a good engine....lots of torque and it has potential with a few very minor mods to get some good MPG......go visit this site and check out their tech tips section for some goos ideas:
And if the truck still dont have the power your looking for then check out their Twin Turbo kit!!!
I wish they had this kit for the 5.2!! I'd slap that thing on my Jepp Grand Cherokee in a heart beat!!!