LSD, Posi-Trac, welded spiders, locking diffs, all are legal and all are fundamentally the same thing and they are all legal on the street. Back in the day most cars came stock with posis but nowadays it is an option in some cars and not available from the factory in others. Most modifications are legal with the exception of nitrous and anything that lowers your vehicles overall safety. Rollcages, strut braces, wider wheels, aftermarket suspension, bigger brakes are all legal and actually should be encouraged because they offer superior performance to factory parts and increase a vehicles overall safety. And hell even if all that stuff was illegal I'd still drive my Z.