Dual cases: you will need a dual case adaptor and another low range gear box (the easy part) you will also need new driveshafts (front and rear) as well as modifying your transfer case mount. Also, you will need a speedo cable extender if you have a mechanical speedo. make sure you get a top shift transfer case as you will be unable to use a foreword shift model for the rear case. There are also details relating the the spline count of the rear input shaft that are escaping me in my pre-coffee morning here.
The crawl box is merely the addition of a second case, either case can technically be called a crawl box. However, the rear case is usually modified with a set of 4:1 gears, thus earning the name "crawl box" but it can be a stock case too.
Good luck! dual cases are the best thing to come to rock crawling!
