I'm not an XJ owner, but know several who have them - both stock and modified.
From what I've learned from the modified rig is that you're best leaving the cooling system alone. One guy spent $$$$$$ on several aftermarket radiator / fan setups and eventually went back to the stock cooling system after a couple of years of outright failures or marginal results. Seeing the gauge in the red on a -10 degree day driving 45 mph on a level maintained dirt road isn't any fun. And yes, he took other measures to make sure his guage was correct and it was. And the radiator was new, as well as this huge electric fan powerful enough to create waves on the ocean that surfers would die for ...
The others with modified XJ's did their homework and took the advice to keep their systems stock.
A friend of mine has a 1991 XJ, with over 300,000 miles. It's totally stock. The engine / drivetrain has never been overhauled. A few years ago his XJ started a gradual progression to running warm, then hot. He took it into a LV shop and found that it was a combination of the fan clutch and burned out auxilliary electric fan. Both replaced and his XJ is cool once again.