Something like one of these....

http://www.amazon.com/WA-7500M-500-...me-garden&qid=1275931785&sr=8-25

These portable units have a hose used for the exhaust air and moisture. If I ever go this route, I would build a shroud that fits one of the side windows in the cap and plumb the exhaust to it. 7,500 btu is probably overkill for the back of a truck but it's portable and pretty easy to work out.

Alternatively, you could get one of those really small window units and build a wood mount for it that would also block off the tailgate opening but with a small door to one side (tailgate would remain open).