Hey! I just picked up my first Suzuki Samurai and so far I love it, but I want to go bigger tires on the stock axle. here is a little info for you. It has chromoly shafts and 5.38 gears and they are locked front and rear. Thanks for the help guys!

 31x10.50x15 Is the max tire size I'd run with those axles set up the way you describe....
I'd recommend trussing the axles even with that tire size.
 You could go larger depending on what type of locker you will be running.
An ARB type locker will allow you to disengage the unit while in a maximum traction situation and will save the axles.
 In that case you may get away with a 33" tire for a while.
 A Spartan style locker or spool won't . You will be in danger of braking axles with a 31" tire.

I speak from experience ....

I definitely agree on trussing the stamped sheetmetal axle housings.  IMHO many of the broken axles/locker issues are actually due to bent housings.  Been there, done that.

Once the long axle side of the housings are trussed, and 26-spline front axles are installed, a tire size up to 33" can be reliably run with locked Samurai axles.  IMHO anything beyond 33" is really Toyota axle/Dana territory.  I also agree with running an ARB locker-it will take a lot strain off the drivetrain when traction is not needed.

Finally, with 33" tires you will need to be careful on how much power is applied in various situations.  It will be near the limit of the axles, but if you don't start hopping on rock ledges, jump the Samurai, etc etc they should last a reasonable amount of time.