Around here, (central alberta) chain drives like what you describe are quite common on large equipment for winches etc. Try places like BC Bearing for the gears and chain or possibly try a truck rig up shop like Danco supply or maybe a shop that works with PTO equipment like Pat's Driveline. A little yellow page reading should give you a few alternate choices.
Your idea is sound for what you want to do and will act just like a divorced t-case.
Doing it this way would allow you to use a engine/trans out of any 2wd rig.