After a lot of miles, another source of leaks is the halfshaft seals in the side covers. They're a bit of a pain to do since they go in from the inside, so the iron cover must come out first. And if you DIY, don't cheap out- use genuine ZF side cover gaskets. Far-east 'equivalent' gaskets may be a different thickness, which will change the ring & pinion pre-load setting.
The reassembled ZF may not leak, but preload changes can make noise when under power, or more rarely when backing off the accelerator.Then you get to re-shim the side covers to reestablish the proper pre-load, which is kind of a pain. This also sometimes happens when replacing the ring & pinion gears for a different ratio.