Curious to hear what you find here. When I was redoing my front diff seals which I had open for other work I found ton of postings on other X drive cars where the front diffs were blowing up often. I have not seen much posting of it here on X5s though so maybe not as common?
I suspect you are way past this as a help but on my recent refill of diff I did add Liqui Moly 2019 MoS2 Anti-Friction Gear Lubricant per suggestions of a few places. Cannot speak to how / if it helps drop noise since mine was not prior.
http://www.amazon.com/gp/product/B00CPL8UPY
As ref I used redline 75w gear oil
http://www.amazon.com/Red-Line-57904.../dp/B000CPCBF0 in front and rear with that Mos2
Curious to hear what you come up with on front diff options too.
J