For some obscure reason I read a few months ago but forget what it is, there are a number of engines... Jeep included... that don't run well on Bosch's Plus4 and Plus2 plugs. Certainly lots of negative experiences with them on the 4.0L and 2.5L. I did a Yahoo search and found lots of discussions on the problems with the Plus4 and Plus2 plugs so Jeeps aren't the only brand that don't like that particular plug. One of those discussions had a Bosch engineer's comments on what the likely issue was for some engines, I wish I could find that thread now but I can't.
For me, I just avoid all such gimmick plugs which include the Plus4, Plus2, Splitfire, Iridiums, etc. I've had great luck with my Autolite Platinums but you even need to be careful with which platinums you use, if you want the extended life platinum plugs offer. Newer distributorless 4.0L engines are fussy and can only use either double-tipped platinum (platinum on both sides of the gap) or conventional non-platinum plugs. Using a single-tipped (platinum on just one side of the gap) platinum plug on a newer distributorless 4.0L engine will cause problems.
Older 4.0L engines with distributors can run single or double-tipped platinums without problem though there's no benefit to the more expensive double-tipped platinum plugs on that type of ignition system.
