Heater core question?

Is it a big job on a "97-up XJ?

It's non-trivial... The underhood part is easy, but in the cab, you'll need to take apart the dash and pull out the box for the a/c condensor. You can finagle the core out without removing the condensor. It took me the better part of a day, but I work slow...

Define "big". I would say big=Yes. Complex=No. Label everything. Take pictures. Search. You'll find several good writeups.

If you're going to change the core, change the evap as well. Replace all the foam.