I had my rear differential looked at last year. Initially, the mechanic thought it was a rear wheel bearing. Once it was on the hoist, he realized it was not the wheel bearings and I had him remove the cover on the rear diff. The bearings in the diff were cracked and needed replacement. But I also had the metal shavings in the fluid. Apparently, Nissan will not admit it, but the hardening treatment on the gear teeth is sub-standard and breaks down resulting in early wear of the gear teeth.
The cracked bearing in the diff were replaced and everything was tightened up and has been ok for the past year. But the gears in the diff will need replacing eventually. There are more robust aftermarket parts available for this problem, which I'm looking into now.