[Mike C]

If your truck has the Dana 44 rear axle it will be on a tag unless someone has removed it. Otherwise, get the stamped number off the top passengier sid axle tube ~6" from the pumpkin. Or post what motor your truck came with. Most likely it has 3.73. You can pull the cover and look for the ratio numbers stamped on the ring gear (39:11, 41:10 or something similar.) After 35 years it probably needs a fluid change anyhow...