Two more things.
Usually when the seal hardens, it cuts a groove into the mating surface on the trunnion. If this isn't taken care of, the new seal will leak. Get a Redi-sleeve to go over the trunnion, cost is $35-40.
Also, be sure your differential vent is clear. Location varies with the rear axle housing, on the 3/4 ton driver's side on top of the housing. It's probably under a glob of gunk. It unscrews and is easier to clean off the truck. If the vent is plugged, pressure from the heated gear lube will blow out the weakest seal, causing another replacement.
