Least expensive would be a another 2.9l.
Best power, at a reasonable cost, would be 1990-1994 4.0l engine swap
You need the computer and all the wiring in the engine compartment, including air filter box with MAF sensor from the 4.0l vehicle.
2.9l and 4.0l have the same block design so current transmission and motor mounts will match up.
2.9l will have slightly better MPG than 4.0l.
302(5.0l) V8 swap requires new transmission and mounts, plus all the new wiring, computer and engine compartment bits.
For better MPG in general you can change the gearing in the differentials for your needs.
Look on the drivers door label, at the bottom will be the AXLE code
Look here to see what you have: http://www.therangerstation.com/tech...le_codes.shtml
Find the RATIO, i.e. 4.10, 3.73, 3.55, ect.......
4.10 = better low RPM torque, for pulling trailers, but at 65mph RPMs are high so MPG is low
3.45 = less torque at low RPMs but higher top speed and lower RPMs at 65mph so better MPG
3.73 = the compromise Ratio, most common, can pull OK and has reasonable MPG at 65mph
The lower the number the better the MPG at speed, 3.08 is the lowest stock ratio.
4.56 is the highest stock ratio, you wouldn't drive this at 65mph, well you could drive it at 65mph but just to the next gas station, lol.
Since you have a 4x4 BOTH differentials need to be changed.