Right now no one manufactures an adapter to do a bolt-together doubler for the Nissan TX-10A transfer case. If you want to keep the nissan engine and transmission you can purchase an adapter from Northwest Fab to allow connecting a "standard" six-bolt round pattern transfer case (options of NP-231/241/242, Atlas, Dana 300). There are a couple guys spread across a few boards that have built their own doublers using mostly Nissan parts. To do it you have to hack off the front output section of the front transfer case and weld a split cover plate over the gaping hole. The stock transfer case is a 2.02:1 ratio, so you can get a 4.04:1 but the assembly will be freakishly long as the Nissan powerplant/trans/t-case assembly is huge.
The biggest issue is adapting the shafts to work if you start mixing transfer case types, all the plate adapters are fairly straightforward. The Nissan 4-cylinder transmission output is 24-spline and the transfer case is 30-spline. The V6 transmission output is 30-spline and matches the transfer case, so technically the output of one transfer case can slip into the input shaft of another V6 transfer case.
You can always purchase some AC (Rockhopper) 3.7:1 gears or Calmini 3.92:1 gears for the transfer case to get a crawl-worthy low-range. Not cheap either but simpler in execution and does not require custom adapters, changing your driveshafts, or creating custom linkages to shift the rear case.
