For those of you who still have the viscous fan clutch, I have some info.
The 2.5 TD and 5.9 equipped ZJ's have a different fan clutch to the 2.5 TD XJ's, apparently heavier duty. Looking through the Hayden catalog I've found that the XJ OEM fan clutch part # is 2771 and is listed as Heavy Duty. After more searching I found that there is a Severe Duty alternative, part # 2791. The Sever Duty version is thicker and a bit smaller in diameter. Both are available on Rockauto.com and the price isn't too dear either.
New clutch is Hayden 2791, old one is what came stock on the car:
