If you have ESOF, this seal is also a vacuum seal. A vacuum leak here can either keep the hubs from locking automatically, allow dirt and water to be drawn into the spindle bearing or both.
Replacing these seals is part of the spindle bearing service. The recommended service interval for these bearings can be found here https://www.fleet.ford.com/maintenance/maintenance_schedules/Getymm.asp
Replacing these seals is part of the spindle bearing service. The recommended service interval for these bearings can be found here https://www.fleet.ford.com/maintenance/maintenance_schedules/Getymm.asp