you NEED gears :read:
i honestly do not get why people put big tires on and leave 3.73s in. its hell on the tranny.
since your truck isnt an offroader, i would consider 4.88s myself. 4.56s will be fine, but 4.88s would be optimal as far as returning your RPMs to the correct power band.
the tuner will not do anything but get your speedometer in the right spot and the tranny to shift at the right time. your tranny will still blow prematurely if you just get a tuner.
and like adam said, your stock gears are either 3.42 or 3.73. most likely they are 3.42's if you have a V8 and no trailer tow package.
i personally have 3.73's with my stock 31 inch tires and i feel that it is UNDER-geared, i think 3.90 or 4.10 would be a much more practical stock gear for 31s :bawl: