[Trish]

I think I can help you.

The problem you are experiencing is a simple fix, The Spring washers you use as a four can flatten and should be replaced when you notice the diff starts to undo itself. When they are flattened and the diff is put under load the springs would normaly compress if you have gone past its threshold but as they are flat and you keep tightening it up the bolt receives the grunt and snaps.

I had the Same problem and when i changed the springs which is the cheapest part of the diff I had no problems afterwards.