posi = limited slip, they are essentialy an open rear end but have disk or cone style clutch packs to keep friction on the rear end. this friction keeps either tire from freewheeling from lack of traction and keeps power going to both tires. as long as the clutches are in good shape they work fine. but when the clutch packs wear out you have an open rear end again.
a locking rear uses two meshing faceplates to physically lock the two axles together in strait line performance, the system then unlocks to go around corners and operates like an open rear end.
the improoved version of the locking differential is the geared rear end, like the detroit truetrac or zextel torsn unit. this uses a geared drive that actually sends power to the tire with traction rather than the one thats spinning.
and then there are spools. which eliminate the torque biasing and tie the two axles together perminatley, this is a drag only option, as the two tires cannot change speeds to go around corners.
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