D21 and other have two factory options (for the rear)
the lower option is a c200 style, similar to a R200 from a silvia/skyline.
the higher option is a h233b unit, larger unit/axels etc.
Both LSD units can be re-shimmed to be tight again, or over shimmed so its more like a locker. This is a good option for any worn LSD's.
if you want a locker or air locker etc etc - you will need an open diff to remove and replace the center with.
with the front diff, you will most likely have a r180 style open diff. Its very rare to come across a R200a front diff, this is usually found in the v6 pathfinders/terranos.
i think locka recently release a locker for the r180 style diff, but these are the weaker option. if you want a front locker or LSD then look at doing a r200a front conversion.
This way you can get a locker, that will be less likely to break - or its easier to get a LSD center for it.
regards,
Christian