You don't need solid axles to do well off-road - unless you want to do some serious bouldering.
When I ran the rubicon we were with someone who had an ML320 with a mild lift and large tires and he could run most of the same obstacles we did in our lifted CJ7s, and he made it to camp with us too. I'd call that capable and I'm sure an X5 outfitted in a similar fashion could do just as well.
Here's a pic of my X5 now with some 18s and snow tires - they look like serious off road rubber and I bet I could get into a lot of trouble with this thing before getting stuck.
