You say you want more highway speed. Good. What gear ratios do you have now in the diffs? I don't know these trucks real well, but I would guess probably either a 4.56 or 5.13, likely the latter due to the 4WD. How tall are the tires you plan to run after the rebuild? Tire height can greatly affect highway rpm. Find an online calculator and look at the difference between a 30 inch tire and a 36 inch tire. Once you get that figured out, then decide how you want to approach higher gear ratios. You can replacthe the gears to a higher ratio, numerically lower, but you'll lose low speed torque. Then there are overdrives, both internal to the transmission, like a T5, as well as seperate such as a Gear Vendors unit or Ranger. Research them and decide what will work best for you. There is lots of information out there if you know where to start.
Devin
