So I just came back from a ride, in which I measured the speed and the RPM during 70MPH. My tires are 245/75R17, and they are aired down to 28 PSI.
At 70MPH, in overdrive, my RPM was at ~2050. This is with my stock axles. My understanding is that 31.5 inch tire, when aired down to 28PSI under TJ, will be really more like 30 inch maybe.
New axles that I need to install with 4.56 ratio, will bring up the RPM by 22% (4.56 -3.73 = 0.83, 0.83/3.73=0.22) so overall increase in RPM will be by RPMx1.22. In my case it will be 2050*1.22= 2440.
2440 RPM seems to be great RPM, and I believe that will drive much better.