You can go one of three ways. One, change the trans to an overdrive, two, change the rear to a higher gear ratio, three, change both the trans and rear gear. Doing the trans swap will probably give you the best overall, though that might depend on what gear is in the rear as to how much you would actually gain.
A gear swap in the rear can get expensive, or, depending what rearend you have, virtually impossible. If you have the Eaton that the 3/4 and 1 ton trucks originally came with, there's not much in the line of available gearing, the 3/4 ton 4.57, the 1 ton 5.13, and the '68-'72 3/4 ton 4.10 (rare). However, if you have a later rearend, like a Spicer 60 or GM 14 bolt, that opens up far more choices for gearing, as parts are readily available for both.
If you do have the Eaton, and want to swap to a higher gear rearend, the Spicer 60 from an '80s-'90s Dodge or Ford van will work fine. The common gear in those is a 3.55. You will need to relocate the spring mount pads.
