I'm sure you have already done this...but did you check the owner's manual? In that little zipper book thing that it comes in, there used to be a card that had the code on it.
As far as your tires go...I think the stock ones on yours are like 275's right? I run on Matagorda Beach with my truck all the time...There was one time that I got stuck, and it was my own fault because I was trying to push it and go through something that would have given a jeep a run for its money. I had 265/75/16 on there as stock and when they wore out, i went up to 285/75/16. That bigger size def helped, but if you are really in a pinch, just let the air down to around 25 lbs to help spread your load out.
All that being said...your stock tires will get the job done, but bigger will always be bette. Your truck should be a tad lighter than mine, and you def have much more power stock than I did so I would go for it.
