Hy panmanhd, the preload on that particular differential is set by a spacer between the two tapered roller bearings, so all you can do is tighten up the pinion flange nut up to 160 - 280ft.lbs., use some kind of tool to hold the pinion flange, a large pipe wrench braced against the frame or ground, or whatever you can think of. Do not use an impact wrench using the ring gears and axles to hold the pinion, the impacting shocks can break or crack teeth, hope that helps.
