The FF has a hub that sticks through the rim about like a front axle, but without the lock-outs. It has a bigger ring gear and is stronger as the axle does not support the weight of the vehicle. You will see the bolts that hold the axle in.
The SF has a hub that sticks through the rim just far enough for the rim to ride on it, like .25" or so. It uses a 9.5" ring gear and is not as strong for a number of reasons. Many of these were 6-lug. They are a nice upgrade from a 10bolt rear on a chebby truck as they bolt-in, but people want more bucks for them than a FF even though the FF is a superior axle.
If you have to ask these questions, ask them in newb 4x4. Even though this is General 4x4 discussion, it doesn't really mean "ask anything."
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