I think it's part of the hydraulic jack that came with the truck, the bit you turn. Is that all it is?
(http://img219.imageshack.us/img219/4734/ehh8cm.jpg)
That's awfully long to be a crank for a jack...
I would say that's what it is if it was, say, a pickup truck with a spare tire mounted under the bed. My '81 Scottsdale has a long tool like that mounted in the engine bay (it actually folds in half and is stored under the factory jack, under the hood, on the passenger side in front of the wheel well), since the factory spare was mounted under the bed. You had to use this tool to release the spare from its mount, since there was really no other way to get to it without getting on your back and sliding yourself under the truck (which isn't hard to do in and of itself, and which you ended up doing anyway to retrieve the spare...).
But, obviously, your spare is mounted on the rear door. Which makes a tool like that just... retarded.
You got a owners manual?
We had a F-150 that had the jack hidden in various places under the hood <_<..made me so mad trying to find the dam thing we you were in a tight spot
The crank for the jack that's in my friend's Explorer comes in three pieces and is about 3 to 4 feet long when assembled.
One end of the thing is a bit flatter, and it seems to mate with the turn-y bit on the jack...
I don't think the owners manual is any help.
Part of a tire iron for real he-man jobs?
My physics teacher's favorite tool in his garage was his 6 foot long hollow metal tube. Torque is a cross product, after all.