excuse me...i think some other work other than a gear change is needed. i suspect that engine isn't being taken care of.
but onto your question...
is the cheapest place for high quality gears. They carry only Yukon gears and if you order the 'name brand' gear set....the cheapest on the site....you GET yukons. I did this and saved i believe $80 on gears.
Mileage comes witht keeping your engine in it's torque curve. AND...depends on your style of driving....and where you are driving. Do you do mosting city driving or highway driving?
I suggest 4.11 for your current tire. 4.56 imo would be too much gear for the tire...you'd be chugging fuel more on highway speeds, but you'd also get better in-city mileage.
First and formost.....get that truck running right because there's no effing way a ranger gets 6mpg.