It is my understanding that any 4.0 with distributorless ignition has the 0331 head. The 0331 head is the only casting that has the mounting boses for the coil rail. So, that would mean the first 0331's were produced sometime in late '98 for the first WJ Grand Cherokees and by late '99 all 4.0's were getting the 0331 casting because all 2000 models were distributorless. Very little, if anything, changed until the very last 4.0's were built earlier this year for the last run of TJ's...the end of an era has passed.
I know a lot of people hate the 0331 head because of the cracking issues. But there are literally millions of that head casting on the road and on a % basis, the problems are pretty small considering how many are still running fine every day.
Knock on wood, hope mine doesn't crack tomorrow,,,but if it does, I'm going aluminum!