The hard part is putting things back together without damaging the new seal. If you take the brake drum off the hub, it is a lot easier to slide the hub straight onto the shaft. There are two slot screws that hold the drum on, usually you need an impact wrench to remove them. If you try to put the hub on with the drum attached, you need to set it on a floor jack and slide it straight on to the shaft.
You will probably need to put a redi-sleeve on the differntial shaft. If there is a groove worn where the seal seats, a new seal will soon leak. You can buy them when you get the seals, they are available at most any parts store. Chevy used the same seal up into the 70's.
One more thing, there is a vent on the top of the differential housing that needs to be cleaned. It is on the driver's side and is threaded into the housing. If the vent is plugged (and most are) the oil vapor will expand when it gets warm and blow out one of the seals.
