Hi and thank you for using just answers.com, I'm Reid and i will help you with your question.
This is a fairly common problem with thies trucks and i dont know y the ford dealer wants to replace the entire pump. Most lilely it is just a bad o-ring or gasket on the pump it itself and they are replaceable. I even found a service bullitin on the issue that you are having. I would highly recomend bringing it to a second dealer and getting a second oppinion.
LEAK - OIL LEAK AT ENGINE VALLEY PAN AND/OR
HIGH PRESSURE OIL PUMP - VEHICLES EQUIPPED
WITH 7.3L DIESEL ENGINE FORD:
1994-1997 F SUPER DUTY, F-250, F-350
Article 97-2-16 is being republished in its entirety as part of a consolidated 7.3L diagnostic aid booklet.
A high number of high pressure Rexroth pumps and Injection Pressure Regulators (IPR) are being replaced due to oil leaking, when all that is required is a new gasket or seal on either the pump or IPR.
If a suspected oil leak is being observed from either component, replace the pump gasket with new gaskets. Pump or IPR replacement is not appropriate for an oil leak. Any pumps and/or IPR's returned must have location of oil leak noted on the diagnostic form which must accompany the returned part. Refer to the appropriate Service Manual for specific details. NOTE
HIGH PRESSURE PUMPS RETURNED FOR OIL LEAKS THAT ARE FOUND TO BE FUNCTIONING PROPERLY WILL BE RETURNED AND CHARGED BACK FOR WARRANTY.