I am going to be running  31" tires soon and will be changing my ring and pinion gear so i can keep 5th gear. My question is, is a 5.38 in tje rear too low for 31"? And also when changing ring and pinion do you have to change front and rear or can i just do rear?

You must change both to use your 4 wheel drive, otherwise
you will break things.

5.38s for 31" tires is not needed for a Samurai, do you have
a Samurai or a Track/Kick

Wild

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.html
this 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

My Samurai runs great as a DD with 30" tires, 4.625s and a stock T-case.

It performs good as an offroader - which is to say it is much better than stock but not as good as a hard-core wheeler (which I ain't) would want.

Also, try the Samurai gear calculator at Ack's FAQ.  It has the major R&P and t-case ratios programmed into it along with an MPH calculator that allows to find the MPH with a specified tire and engine RPM.

http://www.acksfaq.com/gearcalc-01-04-07.php

I hope that this helps!

thanks alot for all the info guys, i used the calculator web site and found out that 4.57 will put me closest to stock. oh and i have an 88' samurai btw. i will be sure to take lots of pics while doing the lift and everything for you guys. 2" shackle reversal. 2" body lift. and 4.57 gears should work well together right?