For that application, we can definitely put a gear drive case behind your 6 speed. Unfortunately from there you would have to run either a Lefty or a t-case from a 5 speed Taco or from an auto. There is currently no adapter to go from a gear drive case back to a 6 speed t-case.
The next problem we run into on an FJ Cruiser is the gas tank. The added length of running a crawl box will push the rear case back into the gas tank. We have seen 2 ways to get around this problem while retaining emissions legality. The first consisted of cutting and welding the tank. It's a poly tank so it requires a plastic welding kit. That one as far as I know did not work and had a bunch of leaks. The second way is to heat the tank and re-form it to clear the case.
So it can be done, there are just a few things to work around.
