The "early" XJ's (to 1990) used a Delco internally-regulated alternator - Delco SI 1984-1986, and Delco CS 1987-1990.
The "later" XJ's (ChryCo production, 1991-2001) use a Nippondenso externally-regulated alternator, with the regulator provided as a circuit in the PCM. The regulators rarely (but not never!) go bad in those. Replacing the regulator entails replacing the entire PCM (which is why I'm not fond of the idea) or using an early external regulator (and ending up with a persistent alternator code, due to OBD,) or converting to a "self-exciting" Delco unit (also ending up with a persistent alternator code.)
Second that you'll want to check for alternator output voltage at the battery terminals before you suspect anything - ideal charge voltage it 13-14.5VDC, with up to 15.2 being acceptable. However, battery life is shortened if the charge voltage is much over 14.25VDC.
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