I recommend you read this article. It is one of the best I have found on the ZDDP/detergent/flat-tappet issue:
http://www.kiwi-indian.com/pdf/EPQ409_36-39.pdf
While you have never put any ZDDP additives in yourself, it was used in your engine for the initial break-in period from the factory. The Mopar cam/lifter break-in additive is used for the first 1000 miles of a new flat-tappet engine, and it can be left in until the first oil change. Depending on the year of your XJ/4.0 engine, it likely was living on ZDDP for most of its miles.
No, the 4.0 isn't a high-performance flat-tappet engine, or a tweak British/Italian classic engine. Those engines are more prone to problems with low ZDDP oils throughout their service lives than the 4.0 engine is.
I run Shell Rotella T 10w30 in my 90's 4.0 engine for a number of reasons--first and foremost it keeps the guts of the engine cleaner than any non-diesel engine oils would; the second reason is it still contains a reasonable amount of ZDDP; the third reason is how it stands up to heavy load situations--as in off road use.