Hy windmillguy, once you identify which brand and series of differential you have, you should be able to find a rebuilt diff. with a ratio that will get you more top speed, the supplier may want your old diff as a core to exchange for the rebuilt diff.. Do you have a single speed or two speed diff at the moment? One other swap to consider is installing a transmission with overdrive as high gear, the NV4500 might be a candidate for that, but you would have to check the GVW of the vehicles they were designed to be used in, hope that helps.