The 2 boxes have the same ratio.
Basic differences are the mounting holes as previously mentioned and the sector shaft on the 2wd is smaller. The pitman arm is almost flat on the 2wd box and is about 1" shorter than the 4wd pitman which will cut down on your turning radius significantly (I was able to take the steering stops in about 3/16" inch after switching). Since the sector shaft is smaller, the pitman arms will not interchange. The taper on the 2wd pitman is also correct for the FJ TRE's that Marlin sells.
Other than that, they work OK. I had one on my truck for about 6 months. I used it cause that's what I had when building the truck. However, I would not do it again. I developed a leak (not cause it was a 2wd box, but cause it was just wore out) and swapped it for a 4wd box I picked up. had to cut all my frame plating off and re-do everything (hack core support, new plates, lots of grinding/cutting/welding). it was not as much fun the 2nd time. Do yourself a favor and just use the 4wd box to start with.