So here is (was) the problem.
According to the SM at the State College, PA dealer, this part had failed some time ago on my X5, but it didn't seem to affect the engine at all. Then the other night a valve in this piece actually broke off and caused a vacuum leak, which caused the engine to run too lean and misfire. They didn't have the part in stock and I couldn't wait another day, so they removed it completely and somehow bypassed it. I have an appt with my dealer shortly after we get back to have it fixed properly.
Engine runs fine now, though. Not sure what this part does if it runs fine without it.
Thanks to all who replied here. I appreciate the input. I was dreading some seriously bad news about this, but it turned out to cost me only some time and less than $200.
