The Con-Ferr that All J's sells is modified to be able to handle a tire. They install a strong floor in the rack and bolt the rack to factory mounting points via a nice set of cross rails that the rack is welded to. Removing a 60+ lb. spare tire from the roof is pretty easy, (just push it off, but make sure you are on level ground) getting it back up there is when it gets a little tough. I really do not like having the spare on the roof as that extra weight up top is a bit more than I want to deal with on the roof.
I have the 33" spare for my XJ inside the back (it Just fits) and I am sure that if it had been on the roof I might have gone over on at least one occasion.

Once the KJ is running 33+" tires (SFA), I will be looking into a rear bumper w/ swing away tire holder to keep it off the roof.