Found my answer, courtesy of Peter Connan:
http://www.4x4community.co.za/forum/sho ... ock-Angola
In my opinion, the only real problem with the Patrol's diff lock is the actuation mechanism.
Every single failure I have seen can be diagnosed back to failure of either the vacuum solenoid in the engine bay, or the vacuum pipes between that and the diff, or driver error (applying power before proper engagement is achieved).
The system consists of two solenoids in a single housing, and probably the real reason for most failures is that the two breathers do not have proper filters. As a result, dust and moisture enters the solenoids and they get stuck.
One side keeps the locker disengaged, the other engages it. The switch swops solenoids (in other words, one is always energised). If the solenoid that is supposed to pull the locker out of engagement should get stuck in the closed position (fairly unlikely) or the hose on that one fails, the diff could partially engage, even when switched off.
What usually happens though is that the other solenoid gets stuck in the closed position, so that attempting to engage the diff lock results in only partial engagement.
The sliding collar is splined to the side shaft, and has two large dog teeth that engage with the diff carrier. These are quite large and have a taper for most of their length. The result of applying torque during this period of partial engagement is that these two dog teeth get damaged and the locker cannot engage correctly.
Having said that, this is the first one I have seen where the locking sleeve disintegrated, and perhaps something else happened here.
Once the locker is properly engaged, it is plenty strong, I have abused mine enough to know that.