I don't know if you've already taken your diff apart, but like I said, you need a very large size circlip pliers:
For the 188k diff, it usually first starts wearing at the output bearings. But usually they last at least 300k km, so it's not really a weak spot/design flaw with those numbers.
Interesting fact is that very late style 188k come without a drain plug. (only a fill plug).
But I don't think they've been fitted to e46 cars (mainly facelift Z4's and probably also X3's (these share the same rear axle; e85 z4 is virtually the same car but made into a roadster)
Also the 188k diff's have at least 2 different cores: one for ratio's 3,07 and lower, and one for ratio's 3,15 and above. I don't know if the diesels have yet another core (very low ratio's, for example 2,47)