high res pics of the 2005 quad cab:
Dodge Dakota Quad Cab (2005) with pictures and wallpapers
I would say a belly pan, just because the truck sits high enough that there is a lot of air going underneath, even if you add an air dam. Do you have the tow hooks sticking out of the two front openings in the bumper? probably a good idea to close those off. They might feed the radiator though. A grill block will help with the short trips to the shop; I'll bet it takes 5-10 minutes (depending on season) to warm up on that trip.
Changing a final drive on a 4x4 isn't cost effective (gotta do it twice, hehe), but I'll bet that would be a big help as well.