it limits the slip (speed differential) of the rear two wheels. an open diff will let one wheel (the one with no traction) spin while the wheel with traction sits completely still. a limited slip will drive both wheels, so it will still move the truck even if one wheel has no traction.
I have the TRD sport package and I don't believe it come standard whith the LSD although there is a label on the rear end that says LSD oil. I could be wrong and I will be checking to make sure because inquiring minds want to know. Like mine