The only "BB"s in the differential are the ones in the bearings and in your axle, there are eight bearings -- two for the pinion gear, two for the carrier (which is what the ring gear is bolted to), and two at the end of each axle tube. Since you pulled out the "center section" (which we call the "carrier"), then check the carrier bearings first and look for a missing ball. If the bearings there are good, then the next thing I'd check are the ones at the ends of the axles. If those are okay, then that BB is probably from the pinion gear's bearing and this is the hardest one to get to because of the amount of torque needed to break the pinion nut loose (you have to put the carrier back and wedge something in there to stop the carrier and the pinion from rotating)