Paid off is always better. Also, there isn't a substantial increase in MPG from the 4wd to the 2wd. If gas costs are really that serious, you should be moving away from an SUV anyway. A I4 Honda or something. I suspect you like the SUV space, and why not.
If you really want to save gas, try gas improving suggestions at any "hypermiling" page. As I have noted on here a couple times, SPEED is the single largest factor in MPG. Air filter, oil, AC on or OFF can all make small changes but together they don't equal dropping in speed in terms of saving gas. A general 5 mph speed reduction ALL over in your style of driving will result in almost a 50% reduction in energy needed. Obviously this isn't exact.. I would need specific data to get that; but try it. If you are used to driving one stretch of road at 45 mph, do it at 40, and see what happens. You would be surprised.
Don't do starts and stops.. do coasts. When you are coming up to a light, don't wait till the end and stop. Start WAY OUT. Slow down, but roll, and try to not do sudden stops.
You would honestly be AMAZED at how much your MPG goes up by modifying a FEW things in the way you drive.