I'd go with a trailer, if your pickup is heavy enough to pull it decent. We have a IH straight truck with a 466 and all it ever gets is 5 mpg. I get 12 mpg+ pulling a loaded trailer with my pickup. With diesel at $2/gal it's not that big a deal, but it is still an extra 25 cents every mile....and I would wager that fuel will be closer to $4 than $2 over the next, say, 5 years.

It's a hassle sometimes hooking and unhooking a trailer, but it's a lot cheaper way to move smaller (but still too much for a pickup box)loads. If your loads are often 20K or more, then you need a real truck.