Put the rear axle on jack stands and remove tires and brake drums. Then remove the differential cover and drain the oil. Rotate the gears so the pinion shaft and bolt are visible. Unscrew the bolt and the pinion shaft should slide out. Push the axleshaft toward the center on the vehicle and remove the C-lock with pliers.

Remove the axleshaft from housing.

Pull out old seal using a hammer. Do not damage bearing.

Install new seal by using a socket that is big enough. Make sure it is straight.

Then replace axleshaft and do the reverse of how you took it a part. Refill the oil to proper amount.

More detailed instructions can be found in a Haynes Manual for your vehicle.