I had a great time despite the long drive. The truck drove out great the whole time. The check engine light came on Thursday night about 400 miles into the journey and stayed on until about the 600 mile mark when we'd reached the rally site. I parked and unloaded. About 3 hours later when we grouped for a trail ride around 3pm I cranked the engine and the light was out. Drove like a champ on the trails. Very impressed with the D90 after having struggled with my Disco for so many years.
The light stayed off all Friday afternoon and night coming back on Saturday morning when I cranked the engine. Light stayed on all day and night Saturday (when the engine is running of course).
Then Sunday I got up, loaded up and headed out at 9am and the light was out. Stayed out until I stopped for lunch in South Boston, VA when it came back on when I cranked the engine. It has stayed on whenever the engine is running every since. I'd say I put 800 or so miles on it with the light burning. Must be emissions control since I have not noticed any deterioration in performance, power or fuel economy. In fact, I ran 70mph from Greenville, SC south to Columbus, GA and the mileage seemed to improve slightly.
I have reason to visit Atlanta next week and have made arrangements to drop it off for a look see.