Yes you can replace the pitman arm seal.
Pick up a Haynes manual and it should have the procedure.
Do not be surprised if it starts to leak again. The pitman arm shaft gets a groove and that's what normally eats up the seal. The one on my BII was a salvage yard replacement gear box. You can also get rebuilt gear boxes that have all the seals and wearable parts replaced. Costs a lot more then just a seal, but well worth it in the long run for a warranty.
Do yourself a favor. Go to NAPA for the pitman arm puller, or any good parts house that has lifetime replacement warranties. Ours had a different pitman arm, and needed to be changed out. We snapped two pullers in under 15 minutes trying to get it off. (and if you just do the seal? You still need to pull the arm off).
S-