Check tire inflation. Low air will adversely affect mileage. You have a 91 with ARB, lift, 33x12.5' on stock gears likely, so I'm guessing that you are getting 10-12 with the 3FE. I know, as I was running the same setup. You already have AT tires which tend to do better than MT's for mileage.
Properly gap the plugs. Stay away from Bosch and aftermarket, as the stock plugs will maximize power and efficiency. The engine was designed with these plugs in mind and changing to platinum/split fire/etc. has not helped improve performance/efficiency, based on the trials of many others who have tried. Installing new wires can help with the new, properly gapped plugs. Going to a lower weight oil such as 10W-30 also helps, as the engine has an easier time running with less internal fluid resistance. The lighter weight oil will however, also leak out faster if you have existing oil leaks.
The main ways to gain mileage are:
lose weight (lose the accessories)
improve rolling resistance (lose the wide tires)
smooth out air flow (lose the ARB and lights)
run a more efficient engine (lose the 3FE and put in a 350 like I did.

)
Since you are unlikely to lose the cool accessories, correct tire pressure, ignition wires and plugs, new filters/lighter weight oil, drive the speed limit, don't stomp on the pedal/ease into it, and drive with a tail wind will help your efficiency.