1990 XJ with 4.0L inline 6 would be a high pinion D30, probably with 3.55 gears, thats what you need to know when buying gears.
To confirm high pinion, look at where the driveshaft yoke is, high or low in the "pumpkin". The XJ used the HP30 for most of the build span, 1984 to 2000, only the last 2000 and 01's got the low pinion D30.
You will also want the gear ratio to determine if you need a new carrier. One carrier is for 3.55 and down, the other works with 3.73 and up.
Carriers are interchangeable between high/low pinoin D30, but the gearsets are not.