Stock, yours most likely has a 3.46, could be LSD or could be open. They are pretty easy to find used, you could take your chances on many used ones for the same cost of having yours rebuilt. As for changing ratios, an e32 3.64 would be nice. A 3.91 wouldn't be out of line but anything higher than that is getting extreme. An e34 3.73 will also fit. For compatibility, the most you'd have to do is maybe swap the axle hubs (super easy) and perhaps swap the input hub, be careful not to preload it too tight, 125 ft-lbs w/Loktite is a pretty safe torque.
Run full synthetic gear oil, spec GL-5. 75W90 is usually sufficient. Maybe add 4-6 oz. of LSD additive but most synthetics won't need it.