Early D30's in XJs were disconnect units with 2 pc pass side axle, and a separate caliper bracket from the knuckle. Later axles, up to aroung 99' upgraded to a 1 piece knuckle assembly and a single pass side axle. All the previous mentioned XJ axles are "Reverse cut" (high pinion). The later 99' though 2001 D30's were low pinion units, but had the same knuckles and axles shafts as the other 1pc units. The ring and pinion from a low pinion unit can not be installed in a high pinion axle and visa versa. So, no, the R&Ps are not universal between those axles.
It is possible to swap knuckle assemblys between axles. When you do that between early and late axles, you keep the rotor, calipers, and hub bearing with the knuckle it came from.
-Ron