both front and rear need to be exactly the same. if youre driving a samurai i think that would be a little too low geared with 31's personally. just my opinion. a lot of peeps like it low though, just depends on your style.
http://www.lepayne.com/gears.htmlthis calculator is a nifty too
according it this, while traveling at 54 mph you'll be at 3500 RPM.
after you put in 5.38 on R&P, if you plug in the high RPM you can see your speed at that RPM by looking at the 5th gear speed range.
if you were to go with a 4.16 t-case youd be at 69 mph at 3500 RPM (just like stock) The only difference is you get a whopping low range reduction (80 %) thats 56:1. On the other hand, the 5.38 R&Ps will yield a low range ratio of 44.5:1.
THe bone stock low range ratio is 30:1
just some stuff to keep in mind, oh and i could be wrong about all of this.
GL