Gimmick plugs like those iridium plugs you're asking only give the gains they promise when they replace plugs that are completely worn out. At that point, any plug can produce significant gains. A less expensive but very well proven premium plug for the earlier 4.0L engine that has a distributor as yours does is the platinum-tipped AP985 by Autolite. It will easily last 90-100K miles and literally perform as well as those iridium plugs do at much less cost. If you go to the parts counter and ask for the AP985, don't let the kid at the counter up-sell you to the APP985 which is only needed if you have the newer 4.0L distributorless engine that came along in 2000. For your particular engine, you can't do better than the AP985. Whatever you do, avoid Bosch Plus2 and Plus4 plugs which don't do well for long in the 4.0L engine, as well as those snake-oil low quality Splitfires.