there were three different widths IIRC. The 2000 switched mid year to the TJ axle which used the thinner (by about 3/16s) hubs. If yours is thinner, the rotor will end up rubbing on the knuckle a bit. Solve this problem by ordering 1/4" wheel spacers from Summit for $14.
I bought two sets of TJ axles and unit hubs over the years (before I sold my 30). These are a great source for XJ spares.
SeanP