I suggest you to carefully read this page and bookmark it.
http://www.bmwe34.net/e34main/troubl...will_break.htm
I cannot imagine that the drive shaft has been welded onto the differential.
On my website below there is a tire and wheel calculator, where you can indicate the original combo and the required combo and the speed selection and then you see the difference. It is in German, but you will also find it in Englisch somewhere.
http://www.duetto-club.ch/d_kalkulator.htm