As sosidge said, if the diff is coming loose then it'll be because the nyloc nut is worn out, you need a new one.
As for the screw snapping I have never had that issue and I used to run this car a lot and still run a BD which has exactly the same diffs except for a different pulley obviously. I can only assume it is linked to over-tightening because you are worried the diff will come loose.
Either way, the first step is to replace the nut and see how it goes from there. Fixing any problem is an iterative thing, change one part at a time until you find the solution.
Hope that helps you. Let us know how you get on with a new nut.
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