Likely a bearing failure going to the right axel - My mechanic doesn't think it's a common problem. Replacing the part with a low miles diff from the junkyard, seals & fluid are being replaced. New differentials are very pricey.
Yes, maintenance history is everything with older BMW'S. At that price, you are ahead for when you need to do repair/ maintenance.
I do 90% of the maintenance on two inline six bimmers - a e90 with 62k miles (n51 engine) and E53 x5 with 125k miles (m54).
Ebay is a great source for...