Preload spec is for new pinion brgs. so trying to measure before taking it apart would probably give you a really low reading. I don't think you could go wrong using the spec for the rear diff though assuming you are using new bearings?

__________________
1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)