[davidka]

Sorry to be the minus-man here but Titanium is almost never harder than steel and the hardest ti alloys are nowhere near hard enough for diff plates. It's a great structural metal, especially when strength in high-heat conditions (the SR71 spy plane is almost entirely Ti) is important but it is never used for bearing applications because it has low surface hardness and a high coefficient of friction. Ti-nitride coating on steel might be worthwhile (like gold shock shafts) but would probably just wear off quickly.