Sulfur compounds are part of the extreme pressure lubrication additive pack needed by hypoid gears. Hypoid gears are found in rear-drive differential ring-and-pinion sets where the centerline of the pinion is offset below the centerline of the ring gear- it allows for the driveshaft to be lower and the floor of the car to be flatter. The gears themselves therefore are sort of halfway between normal straight-cut bevel gears and a worm gear. That means that as the teeth engage and release, they brush across each other while under high pressure. Special additives are needed to keep the specially hardened surfaces from degrading under that kind of contact. Its a bit like flat tappet cams where you have a sliding motion and high pressure at the same time, although not quite as extreme as the tappets.