You are right; just a blank plate by the mirror controls. Other than the Low Range shifter, which I think locks the center diff (?), there's no switch to lock the front/back differentials.
I'm just a layman with no mechanical knowledge, so please forgive my attempt at answering this. I just learned what little I know from this "Diffs for Dummies" site... http://www.lcool.org/technical/diffs/diffs.html