I replaced the differential on my wifes e320 over the weekend. Can someone tell me why there are a set of sensors for ASR and another set for ABS.

Seems like typical MB overkill but wanted to know if there is a reasonable explaination. It seems that if the wheel was locked the ASR sensor would know about it.

BTW the old diff was howling - the 'new' one is beautifully quiet.

There isn't... ASR uses an added sensor for rear. Abs uses a single sensor for rear.

Question about that -- there are two sensors in the diff - each one looks at a ring gear ( I believe). Since there isn't any LSD the wheels can spin independently. Like wise the the wheels can lock indepently. You need at least one sensor per wheel.

I'm understanding ( perhaps wrongly) that your saying one sensor if for ABS and the other is for ASR.

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No,
.......... what I am saying is ABS only uses one sensor on the rear , whereas ASR has to use two so they can distinquish the speed of each axle .. that is how ASR computer knows which wheel to apply the brake to. The ASR and ABS systems use the same sensors for system input.
As I said , on ASR they simply added another sensor, so there are 4 total instead of 3 per/chassis...
You will also notice that ASR also has brake pad wear sensors on the rears.