NOT AUTO ZONE! Replacing rear wheel hub/bearing assembly w/drum brakes.
If they're not making noise yet, I wouldn't bother replacing them as a preventative measure. Unless you go with an SKF bearing, which I think is only available separately at NAPA (you'd have to press it in yourself), you're probably going to downgrade.
I would, however, replace the spindle nut. They're like $8 each from the Ford dealer. Those nuts are laminated with no cotter pin and they are only rated for 4 removals. Odds are if you've taken it in a few times, someone removed them to inspect your brakes and didn't tell you. If these back out, your bearings will fail pretty quickly.
Rally with your goggles on!
2000 Pitch Black ZTS Auto (230,000+ miles)
2007 CD Silver ZX4 SES 5spd
Zetec Crew #190