I currently have 30" tires on my stock Comanche, and I've done trails 1, 4, and 5 in Tellico without any damage. I went with the International Full-Size Jeep Association (in the MJ because my J10 couldn't make the trip) and there were stock GWs on 30s that did the same trails. Parked side-by-side I had 2" more clearance at the rocker panels than they did.
Last year I did Uwharrie in the same Comanche, with the same tires. I did the easy and moderate trails, again with no damage.
I now have a 3" lift and 4.10 geared axles and lockers waiting to be installed on the Comanche, but that is besides the point.
Dirtdogger- your fuel mileage may not be as bad as you think. Since the 31s travel further for each rotation, you are going faster and further than the speedometer and odometer/tripometer show. Switching to 4.10 gears with 31" tires should correct the speedometer and restore stock power levels.