Alex, Peter's test for a leaky diaphragm is spot on
I tried to upload two photos showing the chipped away gears inside my Patrol's locker, but the forum replied with a "Sorry, the board attachment quota has been reached.
Any way, the Patrol's locker has a gear with only two big teeth that engage into another with 6 available slots. Even when the vacuum works as intended, this design allows a slipping drive wheel to spin quite a bit before the locker actually engages. This is obviously not an optimal situation. In contrast an ARB locker has about 40 corresponding splines, so the rotation before it engages is significantly less, with a subsequent reduction in stress when it does lock.
The scenario above assumes the vacuum works well. But when the Patrol's vacuum circuit is not working perfectly, the diff never gets a chance to engage fully and those teeth just end up mashing helplessly against each other. This is what happened on mine. By the time your diff has failed to engage a couple of times, it may already be beyond repair
Peter is the mechanical engineer, I'm sure he'll be able to comment further on the pros and cons of the various designs. I'll try to be clever when you encounter an electrical fault