Guys if this is the 4.0 OHV It's extremely likely you have a cracked head or blown head gasket. This is a common problem on all 4.0 OHV's. The tale tell sign is if you are getting a little bit of white smoke on start up.
If would recommend a compression test. You will probably have low compression on the cylinder that keeps fouling plugs.
If replacing the plug fixes the problem for a short time but it comes back, you know it's nothing ignition related, and it's something thats actually fouling the plug. In this case It's almost a 100% certainty that you have a blown gasket/cracked head.
Ways to check for a blown head gasket/cracked head:
1. Slowly loosing coolant.
2. Smoke at start up.
3. Rough Idle.
4. Fouling spark plugs on a regular basis.
5. Inspect the block for coolant leaks around the heads, and pull the spark plugs after they have been sitting for a while to inspect for coolant.
6. Check for coolant for presence of oil.
7. Send your oil off for a used oil analysis. If it comes back with coolant contamination it's most likely a head/gasket.
If it matters I also have a 99' 4.0L that was fouling on #3. Turned out to be a cracked head.