Welcome to the board and
. What rear gears do you currently have? What rear end is in the truck? You have 3 options. Change the transmission, change the rear gears, or get bigger tires. All depends on your budget. So, what is your budget? What is your fabrication / mechanical skill level? Lots of good information from members of this board, so you came to the right place. Hopefully with more info, we can get you a solution. In the meantime do some searching on Gear Vendors, NV4500, overdrive upgrade, and see what others have done.