In the last few years several companies have persued the idea of "crawlboxes" or "dual transfer cases."  Crawlboxes are seperate units, similar to older overdrives of the 60s and 70s to allow slower vehicles to be able to reach highway speeds, except that they are "underdrives." 

Therefore, with a third lever and a unit installed between the transmission and transfer case, you can engage a lever that allows you to have in a sense two low ranges which can make your final gear ratio unbelievable for offroading.

Personally I am not a hard core off roader and with a low first and decent axle gearing I am more than happy.  However, with diesel engine conversions the need for somewhat road-worthy axle gearing is greater because diesels do not have the higher RPMS that V8s and even 2F gasoline engines have.  The top RPMs that are usually available from diesels are in the 2500-3000 rpm area. All Cummins B series diesels are factory governed to 2500 rpms and even with a 5th gear you still reasonably high axle gearing to be able to achieve highway speeds.

So, if you are going to need an adaptor for a transfer case, though in some situations they can be terribly expensive, why not put a crawlbox in place?  You can then get reasonalbe offroad gearing with faster axles that will still allow you to have reasonable highway speeds.  The crawlbox can serve as an adapter in some situations, and in some cases they are universal - with different adapter plates for the crawl box - you can use the same box with a variety of different transmissions and t-cases.

My personal favorite is the Klune V.  I have had several discussions with a guy named Luke Clark who is an engineer and sales person of Klune V and actually sent us (unfortunately a non functional) demo unit which we brought to Cruiser Moab.  It's about 5" long but serves as the adapter between the transmission and transfer case.  The only downfall of the Klune V is it's $2200 price tag but there are other options too. 

Supposedly Marlin is going to make a Crawlbox for the Land Cruiser that adapts to a Land Cruiser 4 speed transmission but I'm pretty sure they already made one for the NV4500.  They have been making "dual transfercases" for the Toyota Mini truck for several years and they are awesome.

There are also crawlboxes that bolt to the end of your transfer case and make no changes in your drivelines or drivetrain length.

A new development is that you can now use the gear reduction component of a NP203 transfer case mated to either a factory Land Cruiser split case (email me for details) or a NP 205.  This page is dedicated to the NP 203-205 combo.  

I actually know very little about it but I have seen one mated to a H55F Factory Toyota 5 speed transmission in Steve Fox's truck. (He is installing a Cummins 6AT into his FJ60).  This is a sweet setup and can even make a stockish truck great offroad.

I'm planning on updating this page until it has adequate information on all of these setups.  If you have info you want to contribute to this page email me at andre@collegeinternetsolutions.com and I'd love to post it.