If you try to start it in gear with your foot on the brake, or the parking brake set, you put a lot of strain on the starter and the battery, but you're not likely to break anything. The real danger in that is if the vehicle lurches forward (or back) and hits something ... or some one.
If you have access to a wiring diagram, it should show you where the safety interlock is. Most likely it's a simple switch mounted to the clutch peddle support bracket, and you should be able to see if a connector has been pulled off.
That having been said, I'll depart from my usual rule of thumb and give you advice not aimed at answering your question: I think you should leave it alone, and train yourself to always check that the shifter is in neutral and the clutch depressed before turning the key.
Why?
For the simple reason that doing so is the correct, safe way to start a vehicle with manual transmission. Not all vehicles have the safety interlock. It didn't become standard on Jeeps until sometime after Chrysler took over, and larger trucks don't have it. Therefore, if you re-activate it on your XJ and get in the habit of starting the engine without disengaging the clutch, there's a very real possibility you could cause an accident if you get into a vehicle that doesn't have the interlock, but you expect it to be there.
JMHO