before you disassemble the diff, mark the pinion, pinion nut and the yoke. A mark that will line up all three components together. You're going to need to have an inch pound torque wrench that measures from 0 to at least 60 inch pounds.
after replacing the seal, put the yoke back on with the pinion and yoke marks lined up. Then put the nut and tighten the nut up and make the mark line up like it was when you took it off. Then using the inch pound torque wrench, see how much torque does it take to rotate the yoke. (the wheels and brake drums should be off the vehicle when rotating the pinion yoke to measure the torque)