I will tell you ever time I've pulled a head and had a valve job, compression goes up, after normalizing for altitude I can go from 135-145 to 160-170. Never could tell a difference in drivability though. I was always pulling the head for another reason, the compressor numbers were never the driving force. I'd pull the valve cover and have a look. Time, heat, and miles and I've since the valve seals dislodged and riding on the valve stems... on 4.0's... If you're burning oil you can try a dry vs wet test which will tell you about the rings. If it changes, MMO or Auto-RX treatment can get carbon out of the rings, help them seal better, and your compression will go up too. While the valve cover is off, if you chose to pull the rockers, you can look at the tops of the valve stems. You want to see a bulls-eye pattern which will tell you if the valves are rotating. If its a line, then there is a TSB to manually rotate the valves to try and fix it. If it doesn't work, pull the head and go for the valve job. A well maintained 4.0 bottom end will go the distance but they still need valve jobs from time to time... Last time I pulled the head on my 01 at 315,000 I was still around 160 or so after I dropped a new head on (to fix the dreaded 0331 crack).