I buy parts from rockauto and advance auto all the time,
The people who are so thick headed with the BMW only parts lets pay a 500$ premium for them, is just beyond me, i agree for CERTAIN items but most wear items not really. These are the same people who will buy $300+ or whatever the cost BMW bags for air suspension, instead of $99 arnotts.
A lot of people need to realize whether you like it or not, so much stuff is made in china now, and most companies just slap a label on it with their premium name and you pay the extra $ for nothing.
Any and all aftermarket parts I've ever bought including cv's have did fine in all the cars, there are alot of good aftermarket brand names you just need to know what they are and where to find the best pricing.
Another prime example, spacers, people always say this and that about ebay ones, i ran ebay 20mm spacers on my Audi b5 a4 i sold 3 months back for over 1.5yr with no issues, and near the end drive it like you stole it doesnt even describe how much abuse i put on it in corners and throwing it around. Most of it is just common sense with parts.
Heres the link, Its for your 2001 4.4 as well, see the top 2 with the hearts? That means those are the most ordered and popular parts for that particular category aka (Cv halfshafts category)
2001 BMW X5 CV Half Shaft Assembly