Only 'special' tool needed would be a seal puller which can be bought for under $10 at an auto parts store.
Imprompu step by step how-to:
Chock front wheels, jack truck up and place axle on stands. once on stand, put truck in neutral and e-brake off.
get drain pan and pop off differential cover. let fluid drain.
while it's draining, remove wheel and remove drum from problem side(personally, I'd replace both sides for good measure)
rotate carrier(what the big gear is bolted to) til you see the 'crosspin retaining bolt'. It takes an 8mm or 5/16(forget which since I use either). remove the crosspin retaining bolt, then slide the crosspin out of the carrier.
push in on the axle shaft and it will expose the C-slips which retain the axle in the housing. use needle nose pliers to remove if needed. Then slide the axle shaft out of the housing. Place on ground with wheel studs on the ground. Doing this makes the seal accessible.
Take your tool and gently pry the seal out of the housing.
Once removed, clean surrounding area and mating surface for the seal using some emry cloth or sandpaper.
Take new seal and gently tap it into the axle housing using a hammer, while tapping it in evenly and not ****ed. I like to use a 12" long chunk of 2x2 wood to soften the blows against the seal.
Use the emry cloth or sandpaper to the axle shaft where the seal rides(if needed if it's rough).
lightly smear a layer of oil on the new seal. Slide in your shaft all the way, c-clip on shaft in same way it was removed(you'll see wear on the c-clip to determine what side it goes on), then pull out on the shaft to seat the c-clip. install crosspin. use locktite on the retaining bolt. RTV the cover, bolt it on. Fill with 85W90. done.
Last step was a blur. lol.
Hope I helped! Good luck!
'01 XLT disguised as a '03 Fx4 Level II.