[burnin oil]

How many lugs are on the rear axle? If it's 10 you need to swap axles. The 8 lug Ho72 can be had with 4.10, 4.57, or 5.14 gears. Even 5.14s should go faster than 40mph. If you don't like those ratios you are looking at swapping in a later 14 bolt or Dana 70 axle.

Edit. I just saw the lug count in your signature. You are going to be swapping axles if you want more road speed. Also price your brakes and that should be enough to justify swapping.