Have you figured what rear end you have in the truck now? Easy way to do it is, jack the rear end up put the truck in n and spin the rear tire (mark the driveshaft first) if you have a open diff in back lift only one tire. Count the number of times the driveshaft spins per 1 tire rotation. That is your rear end ratio.
For work trucks in the Us 3.55 rear w a 5 spd or od is considered high gearing,
