Didn't the early TJ's from 97 till about 00 come out, (auto & manuals) with a 3.07 final drive?
If it helps, my TJ with a nv3550, running 33x10x15 KM2 and 4:10 final drive, sits at about 2100 rpm at 100 kph in overdrive - it drives well and I get good eco (upward of 500k on the open road) but I would like to go to 4:56 because the nv3550 has the tallest overdrive available in a TJ and I want more power... as you always do
Pretty sure this is based on 1 to 1 (fourth gear)
As well said by bonza, unless you can do most of the work yourself and pick up the parts cheap, changing ratios to save fuel is a bit like Jeep and economy in the same sentence.
Choosing a skinny wheel and tire combination that weighs as little as possible when needing new tyers can help, be fastidious! research the actual diameter, width, and weight from as many different manufacturers of 31" tyers as you can because it can vary a lot! - you'll get better eco from lightweight skinny and obviously, the smallest diameters 31" tyers available but how you use and drive you TJ should influence what you purchase.
Frequent servicing, driving under 100 on the freeway, quick shifting the manual, together with lightweight skinny alloy wheel and tire combinations make a noticeable difference for me.