Replacing the pitman shaft seal is a fairly easy job if you are familiar with steering gear repairs. It should take a good mechanic about one hour.
Remove the pitman arm (puller may be required), remove seal retainer, and remove old seal. The old seal can usually be removed with the hydraulic pressure of the system by turning the steering wheel in the direction to pressurize the seal. Install the new seals, washers, and retainer (seal driver may be required). Reinstall the pitman arm in the exact same location on the shaft and refill the fluid level.
If you have to buy the puller and driver, or if you have no experience with steering repairs it would probably be best to have the leak professionally repaired.
Gerald