David, Since it is a 3/4 ton, I'm going to assume you use it as such! I would recommend a high quality synthetic gear oil for the rear end and the same for the front differential! There are a number of high quality oils which can be used and provide quality protection for your gears, bearings and clutch assembly! Also Adding a quality posi lube won't hurt either!
P.S-----Make sure when servicing the rear differential that you inspect the posi-traction unit for a problem which I've seen multiple times---the unit has a problem where pieces of the clutch assembly fatigue and break---but do not fall out of the carrier but instead will stay in the unit but contact the case and the cap causing catastrophic failure of the housing. The hardened steel which breaks off cuts into the cast housing like butter and basically ruins the unit! If you have any questions feel free to call or come by and I'll be happy to show you what I'm talking about!