There should be premad boxes available for your truck. A single 10" sub and around 150 watts will do wonders for the sound of your system with getting "ghetto loud"
Good luck! I'd recommend any of the infinity subs available on ebay, or right now you can get a Kenwood tornado sub for something crazy like $40 shipped from
www.circuitcity.com, though you have to go pick it up because they are out of stock for shipping from their warehouse, only in store pickup is available. Good luck.