2wd vs 4wd o.d transmissions

needsaresto said

So is the real difference just tailshaft length or is it designed to be physically impossible to work as a 2wd trans?

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The main case (front section) is the same on both. The rear section on a 2wd is just an o.d. unit. The rear section on a 4wd unit is the o.d. unit and transfer case. Therefore you can convert either to the other by changing out the rear section.