The rear ball joints ( 33-32-6-767-748 ) seem to be the most common failure in the rear suspension. But they are really buried deep, and about 10x harder to change than things like control arms, so they are often neglected.

They are definitely not greasable, and although they're called ball joints, there is not even a ball in them. They are more like bushings designed to provide the same angular motion that a ball joint would give. Just rubber with metal sleeves. I replaced those on my '01 E53 when the camber was not alignable within limits, eating tires.

When I did that whole job myself, I isolated the source of some intermittent squeaking to be coming from the upper spring mounts. By removing those (had them out anyway when taking the springs out), I cleaned them carefully and reinstalled with plenty of grease on the body side and spring side. Silent ever since doing that almost 3 years ago now.

BTW, I believe some parts can last a really really long time, so I don't generally choose to replace things until they need it. When I was "in there", I also replaced the integral links (even though they seemed to be just fine) since they are buried deep like the ball joints, and they have to come out anyway. I found one of the two rear upper control arms to be a little loose, so I replaced both of those. But the rear suspension forward upper control arms ("guide links") tested just fine, so they are still in there, working fine at 182k miles. It is a trivial job to swap those, so I figure I can do that if they ever do fail.

So if you have tire wear problems, like the inner edge wearing away excessively, I'd look at the suspension, and in particular ball joints. But if you just have squeaking, I'd look at the spring mounts - that's what it was for me. Not sure if air bags have the same problem, but they're surely a lot easier to remove than the springs were.

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2001 X5 3.0i, 203k miles, AT, owned since 2014