This is the thing I get the most questions about, so I hope it helps make sense of it.
The FJ case I used is from an automatic transmission FJ. Things start to get a bit confusing with the VF boxes as they are so similar, the automatic transmission FJ came with a lever operated part time 4wd transfer case, while the manual transmission FJ came with a full time 4wd that is somewhat similar to the GX box.
There are several crawl boxes on the market, I went with the NWF (Northwest Fabrication) Eco Crawler. But there is the Marlin TacoBox, the lefty, the atlas, and probably many more.
My reasoning.
The GX is limited on space in the tunnel, I knew I wanted a part time system, and I like the reliability of lever operated systems. I would have loved to go with an Atlas 4 speed, but well that would be overkill for this build and my pockets aren't that deep.
The FJ case was the easy answer at that point for the main transfer case, also it is an inch shorter than the GX box, which is super helpful for packaging, and without needing an actuator (lever operated) it comes out to be 5 inches shorter near the gas tank.
Marlin is well known and well respected, I'm not throwing any shade at them, but NWF had 4 major benefits for me. 1 Marlin was not building complete units at the time, 2 the Eco Crawler is 1 inch shorter (easier packaging), 3 NWF has a super clean shifter set up, 4 I got in on the presale while they were still in development.
Unlike the Marlin TacoBox the NWF Eco Crawler only has one gear option, it uses a set of donor gears from another VF transfer case for the gears. so you get 2.57 in your main case, and another 2.57 in your crawl box. Marlin uses a different gear setup that allows you to run 2.57 in your main case and up to 4.7 in your crawl box. For my build, I'm going slow enough with a total of 106:1, but having over 200:1 might make sense for others.
So I bought two FJ cases, got a deal at a local junk yard at $150 each, chain tension was good on both. The first one was stripped for its gears, which were used to build the eco crawler
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Then the Main box was gutted, inspected and the output shaft was swapped for the one used by the eco crawler, at this point you also take out the pin that links 4wd to low range so they are independently controlled.
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Then finally the two are bolted together in to this massive box.
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Then swapped with the one in the GX.
I was afraid the electronics would be complicated, but the electronics are WAY easier to trick than I expected, I can happily talk anyone through it, I've coached a few people who just did the FJ swap.
The GX uses a front mounted shifter, and the FJ and the eco crawler both use a top mounted shifter. This is the FAR MORE tricky problem to solve if you want it to look good. NWF came up with a set of shifters that work amazing. Guys have been cutting holes in their cup holders and through the tunnel on Toyotas for years, but their kit uses a stout shifter linkage to make everything line up damn close to the stock location. I still need to get around to making some shifter knobs for them, but from left to right they go, H-L / H-L / 2wd-4wd.
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