I'm under the impression that clutch = mechanical = X way. I think a lot of the Mullet diffs are clutch-type, which is why you always see LSD additive at Pep Boys. I know that my friend's 98 Trans-Am had to have a diff oil change at 10k miles(!).

By the way, when you mention a "locking" diff to an american parts slinger, they will immediately think of the locking diff on their Jeep Wrangler. Those diffs lock by hand by turning the hubs, and truely lock the diff 100% of the time.