just pull the driveshaft and you can pop that seal out with a flat screw driver, buy a new one and use a socket to drive it on. I've done this several times with success. seal costs around 5 bucks or so. Take it to a shop and be prepared to pay big.
Tools needed:
7/16" boxed in wrench or socket, to remove u-joint strap screws
pry bar, to release u-joint from yoke.
flat screw driver to remove old seal.
big socket to drive new seal on (1" plus, not sure of the size, just pick one that fits the best)
good luck,
Jason