What year is your truck?  V6 or 4Cylinder?

IF its a 4cyl, manual trans, it's an easy fix.  The speedo drive gear on the tailshaft of the t-case is easily changed to accomodate your tire size. 

Toyota did offer 31's as a factory option, so you should be able to order a different speedo drive gear from your local Toyota dealership.  If you pull the gear out of the tcase housing, you'll be able to see a number that indicates how many teeth are on the nylon gear.  Simply order the gear with the correct number of teeth, and you should be able to get your speedo pretty close.  (Note...this only works up to about a 31" tire...beyond that it'll always be off a little)

Ryan