Depends.... there is an o-ring inside the retainer that rides against the stub shaft. That is easily replaced. There is also a lip seal in behind the bearing retainer that may also be leaking... and that requires removing the 4 bolt housing/retainer. That can get more exciting IF the reduction gears drop. Best I recall... you pull the shaft then stick a broom handle or something similar in there to hold the gears up while you pull the retainer out and change the seal.
Just inspect it closely before you dig into it and see where it's actually leaking. Most times it will be the shaft o-ring.