You typically have three kinds of differential noise:
With what you describe, the source may well be from somewhere else. Can the rear drive shaft on your XJ be removed and then driven without losing fluid from the transfer case? If so, remove the shaft and drive in 4wd (assuming you don’t have a 2wd XJ). This will help isolate the source of the noise.
Keep in mind that free advice can frequently be worth less than you pay for it.