Read this for all your tcase questions:
V6 transmission and transfer case - YotaTech Forums
Bigmike knows his stuff
"As for options, you can choose to run a single case with a very low gear set, or run dual cases.
Single Case Pros: This is the most cost effective way to gear down. You simply remove your stock chain drive case and replace it with a gear drive case that has a set of 4.70:1 low range gears. You don't have to worry about shifter, drive line, or cross member modifications. It's all plug & play, add gear oil, go wheeling!! It is the lightest option for Rock Crawling.
Single Case Cons: Once you shift into low range, you are stuck, in very low low range. With your stock t/case you can do ~55 MPH in 5th gear low range. This is practical in case you are headed to the ski resort with snow on the road and want to use 4WD low range. But with very low gears in a single t/case, once you shift into low range, your top speed in 5th gear is reduced to ~30 MPH. Honestly this is not practical. No one would want to drive around at 5,000 rpm in 5th gear with other traffic on the road. In terms of off roading, you'll also find that on simple dirt trails, or more difficult trails that open up with flat sandy sections, you'll likely to shift into 4WD high range and use 1st and 2nd gear rather than revving your engine very high in 4th and 5th gear in low range. So it's just an annoyance, in my opinion, to be stuck with normal highway gearing or super low low range gearing.
Dual Case Pros: This is the preferred method of gearing down. You can select between two different low ranges: your stock low range or your stock low range X2. Because gear ratios are directly compounded, if both cases are 2.28:1, then combined they create a 5.20:1 ratio. So you can leave one case in high range (which is a direct 1:1 ratio) and put the other case in low range, and your truck will drive exactly how it does right now. It will be streetable in low range and you won't have to use high range for those faster sections of the trail. Then, when you approach an obstacle, all you have to do is drop the other box into low range and now you have a double-low ranged truck that provides all of the benefits mentioned above: Reduced speed, greater control, and increased traction (static friction supersedes kinetic friction). It's an option that is always at your service whenever the trail demands it. Also remember that since you'll have a 3-speed transfer case, your truck will turn into a 15-speed! Seriously, you will have 15 different forward gears to choose from whenever you need them! How cool is that?!!!
Dual Case Cons: The cons to going dual case is that it is the most costly option. You have to modify your shifter arrangement in your cab (add an additional lever), you'll have to shorten and rebalance your rear drive line, and lengthen and rebalance your front (we typically don't rebalance our front drive lines but if you do a lot of road driving in 4WD then it is required), and you'll have to do something about your cross member situation. You can retain your stock cross member, but you will need to clearance it for the longer front drive line. Usually people choose to install an aftermarket or custom dual case cross member which provides more strength and an additional transfer case mount for increased drive train rigidity and reduced stress on the engine mounts. There are many aftermarket options for this but of course it all comes at a price! Also, the dual case will add about 50 pounds to your truck, possibly more depending on what cross member you use. Your front drive line will be more exposed to rocks since it is longer.
So what to do? It is completely a user preference decision. I always recommend going the dual case route since you'll get all the hard work out of the way and it is the most flexible option giving you more gear choices. But some customers just go duck hunting and are tired of burning up their clutch every season. It sounds like you want to get pretty serious and I see you mentioned running 4.70:1 gears in the rear case (that is an awesome 4-spd case with 20 forward gears!!), so I see you are pretty much already decided on what to do in this regard, but for other readers it all comes down to what are your needs with your truck and do you want to do more work now (go duals), or more work in the future (go duals later)? I say if you go duals up front, and then later decide you want something lower, all ya gotta do is slap in a set of 4.70:1 gears and it's done!
We call the dual case + 4.70:1 gear set the "Ultimate" Crawler. This will take you to a 174:1 to 224:1 depending on your axle gearing. We feel that at these ratios your truck is virtually unstoppable, only limited by traction."
