Was getting a little stumble from the engine while sitting at a traffic light yesterday,so I ran my scanner and came up with a P0400 and P0420 code. I'm hoping by replacing the EGR valve with a new one that the P0420 code may go away along with the P0400. I'm getting a brand new EGR valve off Ebay for only $30. It turns out that the 4.0L L6 engine uses the same EGR valve as late 90's/early 2000's GM 3800 V6 engines.

 

Update : Got the new EGR valve in the other day. Removed the old one and figured I'd have to clean the port. I ordered a new EGR since it only cost $30 and I have two 4.0 Jags so a spare wouldn't hurt and could come in handy. Cleaning the port with a bottle brush helped immensely. The car idles much smoother and it has removed both the P400 and P420 codes.