The Fun Factor is interchangeable between buggy, rally, stadium truck and monster truck. This is why you have three different shells for one vehicle. The basic differences between the options, is gearing, A-arm width and wheels/tyres.
I would not agree that leaving your truck in 2wd is the best way to increase the performance, they handle much better in 4wd. The Fun Factor has a bad habit of bending and breaking drive shafts and diff cups. If you do a quick search in this forum, you will find a thread that shows how to do the diff cup modification. It basically involves reinforcing the diff cup with 10mm compression fittings or 'olives' as they are known. Doing this will toughen the drivetrain up considerably. Anyway, welcome to the forum and good luck with your truck, if you have any problems, everyone here is extremely knowledgable and will be only too glad to help.