There's no such thing as a "4x4 specific" LSD. If you have a 7.5" rear axle, then any LSD for a 7.5" will fit. If you have an 8.8" axle, then any LSD for an 8.8 will fit. It doesn't matter if your truck is 2wd or 4wd.
If you're looking for a used axle in a junkyard, then any Ranger axle will bolt in and work. It doesn't matter if the axle came out of a 2wd or 4wd truck; they both used the same rear axles. LSDs were available in both 2wd and 4wd. Just make sure you get the gear ratio you want.