Just found the whole procedure in the FSM, 05-410 "Removal and installation of helical gear shaft". They make a point of saying in this procedure that the o-ring (there only appears to be one) should be checked and replaced if needed in step 8. Looking at the diagrams, I think you can probably get away with replacement of just the o-ring without having to get into steps 5 and 6.

For any following in these footsteps in the future, the retaining bolt you reference is torqued to 45 Nm and the cap bolt (the thing you see in the picture) is torqued to 50 Nm.

Will keep you posted...

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Garrison R.
'83 300D