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.