Have you guys considered a brownie box? if your transfer case is divorced you could easily install a Spicer 5831 3 speed aux transmission between the main and T case. They aren't that big, but will require the driveshaft lengths changed. IIRC they are rated for like 16K LBS or more.
I have one that has a 2.0 underdrive 1st gear; a 1 to 1 2nd gear; and a .74 OD 3rd gear.
Since you are using manual transmissions, shifting the Spicer would be simple. The shift pattern for the Spicer I have is the same as the old 3 speed on the floor manual transmissions of the 60's. You could just get an old Hurst 3 speed floor shifter and use cable linkage shifters to make it work with your tilt cabs.
I originally got mine and had it rebuilt, to use behind my Motorhomes automatic tranny, but then decided to just build an Allison 1000 5 speed with stand alone TCM. My original thinking was to just use the Overdrive on the highway with my old Allison 542 4 spped. But it didn't have a lock up torque converter in the 542, so very little mpg gains with that one. And I thought, if I ever needed to climb a steep hill from a standing start I'd down shift to the underdrive and run through the gears until it got to the top. I do a lot of rock crawling with my 4X4 rig we nirmally flat tow behind the Motorhome, so we sometimes take the Motorhome for a base camp way deeper than most would try to go with a big coach, to use it as a base camp for the rock crawlers we wheel with.
The AT1000 has the gearing I need now that I've regeared the rearend that's in it.
I can show you guys some pictures if anyone wants to see what I'm talking about. They are in my other computer.
I have kept it thinking one of these days I would build a 4WD FSR Isuzu camper and just use it between the two gearboxes and have a heck of an off road rig. Talk about a crawling gear!
