If you mean locking the diff as in a spool, then there is no wear as there is no moving parts.
Locking the diff as adrian mentions refers to locking the retaining grub screw in the opposite outdriver to the adjusting bolt. This I believe is the cause for your diffs coming undone.
Follow adrians tips above, he's built a few of these diff's and knows what he's talkin bout....sand the rings with 600 grit, build, adjust the bolt so there little to no slip under force.
Apply a small dab of Loctite to the grubscrew and lock it down firmly against the adjusting bolt.
whalla thats it.