Thanks for all your help, still lookin', still cryin' - I feel that main dealer will get his pound of flesh.
More breakers, more web sites, but think I'm more likely to find an Oozlum Bird before I find a 3.91 Diff !!

Tried buy and sell as suggested but the breaking model there is 2003, which I'm ...

Thanks both,

Did try Fabdirect yesterday - very, very helpful but they didn't have one.
Seems like the 3.91 ratio is as rare as hens teeth - Is that because it's the one found in 3.0D and therefore the most sought after ?

I've spoken to a few transmission rebuilders who all say they can't rebuild ...

Well,
Over the weekend my 2005 X5 E53 3.0d Auto ate its own rear diff. At least I think it did, horrible crunching noises, smell of burnt oil and tell-tale seeping of burnt smelling oil at the junction of drive shaft and diff.
I'm no guru - but that sounds like a dead diff.
So, to the business end ...