Mira changed her own XJ spark plugs today.
A previous owner had installed Champion platinum plugs (which I heard don't work very well in Jeep 4.0s), and the gap varied from 0.048-0.055". They were supposed to be 0.035".
The new, properly-gapped NGK "standard" plugs made the 4.0L run much more smoothly, but we still have a slight backfire when revving the engine. We'll have to get it into a shop in the spring to get that checked out.