If it is a 9" rear end, first remove the drive shaft. Using an inch pound torque wrench (dial type) and socket on the pinon nut yoke, measure the amount of torque needed to maintain differential rotation (rotaintional drag) through several clockwise revolutions. Record the measurement. Then using suitable wrench to hold the yoke, remove the pinion nut. Place a drain pan under the rear diff, mark the yoke in relation to pinion and remove the yoke. You may have to slightly tap the yoke with a hammer to release it. Remove the seal and replace it. There are a couple of differant seals used, so I suggest removing the old one first before you buy the new one, that way you can match them up. You will also need a new yoke nut as well. Install the new seal, put a coat of grease on the rubber lip first to prevent seal burn. Coat splines of yoke with a small amount of grease and install yoke alighning the match marks. Install new yoke nut. Holding the yoke, tighten pinion nut taking frequent turning torque readings till the original preload reading is attained. If the original preload reading , that you noted during assembly , is lower than the original reading, keep tighteningthe pinion nutuntil the specified reading is reached. If the original preload reading is higher than 8-14 inch lbs., torque the nut just until the original reading is reached. You will need a torque wrench that will go to at least 225 ft. lbs. If your 3rd member has been repaired or you've had it regeared, you may not need to do allof this as the remove the crush gasket and replace it with a spacer. Then all you would need to do is replace and retorque the pinion nut to about 175 ft. lbs.
How about an impact wrench for removing the nut? (not for installing it)
