The unit bearing has to be replaced as a unit. They do not sell replacement bearings for them.
The most difficult part of the job is seperating the unit bearing from the steering knuckle especially if you reside in an area where salt is used for snow removal.
I used this tool and it still was a chore: http://www.ktoolinternational.com/PDFs/MEGAHAMMER.pdf
Both unit bearings ended up seperating and and an upper ball joint broke getting the unit bearing to seperate from the steering knuckle.
When you get ready to reassemble use anti-seize compound on the mating surfaces of the unit bearing and steering knuckle.