Hy dantheman49-6400, an auxiliary transmission is mounted on the frame, in between the transmission and the rear axle. Some of them have three gear choices, underdrive, direct, and overdrive, some have only two, direct and under, or direct and over. The ones appropriate for your truck are getting rarer as I type, and they are expensive, they also require mounting crossmembers, shift linkage, and custom length driveshafts. Personally I think an overdrive transmission would be a better choice, I think a fairly common one is the NV4500 [I may be wrong on the number] hope that helps.
