Does the pinion have the same number? They usually do to show they are a matched set. On a ZF, the numbers that matter are usually stamped in the end of the pinion and on the housing for pinion depth and those numbers generally appear something like this: 88,0 on the pinion and 119,80 on the axle housing. Once the pinion depth is checked then backlash is a function of just moving the ring gear and that spec is in a manual, not stamped on the ring gear. Most axles would be around .007" - .015" I would imagine. Then tooth pattern also needs to be checked with Prussian Blue.