Right unless you do a SAS. Probably less work to do a SAS than to source these rare Isuzu gears...
Its too bad that 4.56 is the lowest readily available gearing option, its really the only downfall to the Isuzu 12 bolt. IMO if gearing wasn't an issue, I would take a 12 bolt over a D44 anyday, the strength is comparable (as Billy said) & 3rd member axles are much easier to work on.