if you're doing big tires, gearing down your diffs will make it better as a DD because it'll be getting you closer to the stock final drive ratio, which is where that 4.8 will enjoy its life much better. it'll also save you a bit of gas around town.
00 FRC - just top end, exhaust, and a hot-air intake
99 Sierra - 5.3 / 5spd
↓ click the pic for the build ↓