Not sure which forum I saw it on - may have been AUSubaru, but there was someone recently who put some d/r boxes in a container and shipped them for a group of forum members in the U.S.
I believe there was someone who swapped a dual range EJ gearbox for a complete MY hatch in WA a couple of years ago... Crazy! But dunno if it actually went through.
All of this will be do-able. You just need to know what you need and how to get it to the US.
Pack and send has worked well for Giles in the UK (ausubi) who had a brumby halfcut sent to him through pack and send! His last aquistion with the help of forum members was a complete power steering setup for his brumby. As far as he knows he is the only fella in the UK with a brumby/brat/MV with factory AC and power steering!
If you contact scorpion in austraila they might be able to source the gearbox you're after, fit their low range (re-produced L series low range from what I understand) of 1.6 to 1 (L low is 1.59:1), they could check out the box and send it your way to have it fitted. It will cost you though.
What you'll need is the interior console, cable and mounts plus the lever for the hi/lo range, the gearselector boot that fits the hi/lo cable.
AFAIK the single range gearbox is the same length as the dual range units as the dual range tranfer gears are on the input shaft located above the front diff.
I'd look at sourcing the rear diff/axles in the US if they're different to what you have now (to make them match the gearbox), less packaging cost this way.
I don't know if your instruement cluster has the lights to display low range or not... Maybe look into this or have a plan on how you're going to have low range display once you've got the gearbox installed.
Cheers
Bennie