Just start calling auto repair shops if you can't find any drivetrain specialty shops. I think any good shop that would work on such things would have access to an archive for the Isuzu tolerances, backlash, etc. See that they have useable numbers yourself - don't just let them say, "Yeah, we know the tolerances." A lot of people just use GM torque specs and stuff on everything from engine rebuilds to hinges! Maybe you could call St Charles and get the gear setup numbers from someone there, or find them online, and give those numbers to the shop you choose? I am going to take my front end to a mechanic friend next weekend. I guess you could also ship out your 3rd's to a good drivetrain shop if you can't find something local.
Sent from my "two hands on a keyboard"