QT2 (10 bolt cover) I'm pretty sure there are a few options for a solid steel cover, I can't remember who makes them, but I know they are out there. It might also be sold as a 8.25 Corporate axle (I think please verify before purchasing I'm on too crappy of internet to research so going from memory)

QD2 is also a 8.25 axle (or whatever is the correct measurement see above..) but it's 12 bolts instead of 10 to allow for the electronic slip solenoid inside the housing.

For everything else besides the diff covers the skids are the same on all XK's regardless of year or options.