FF means full-floating. SF means semi-floating. That's referring to the axle shaft. A FF axle will have a hub with two bearings that the tire bolts to, just like a front axle. The hub is completely separate from the axle shaft.
FF rears are inherently stronger than SF, since a SF shaft has to both hold the weight of the vehicle and transmit torque to the tire. A FF shaft just transmits torque. The weight is transferred directly from the tire, to the hub, through the bearings to the axle housing.
Where the 14 bolt is concerned, the 14FF and 14SF aren't at all the same. Some axles (Dana 60, 10.25, etc) have both FF and SF versions. The 14 bolt is only called that because it has 14 bolts on the cover. It just happens that Chevy makes two axles with 14 bolts on the cover. Eh?
Long story short, find a picture of a 14FF diff cover, so you know what they look like. Note also that the 14FF has a removeable pinion support (like a 9"), so if you look at the front of the housing, there should be six bolts around the pinion yoke.
Ah, fawk it.... just read this:
http://www.pirate4x4.com/tech/billavista/14b_bible/ it's got plenty of pictures, too.
