53OneTon:
That large bolt in line w/ the pinion is for adjustment of a thrust pad inside the gear case, that is on the "back side" of the ring gear: it is supposed to limit the deflection(distortion) of the ring away from the pinion, when pulling under heavy load.
It should not be tampered with unless you are planning on "setting-up" the whole punkin; although the shop manual does discuss its adjustment.
