Hey Will,
I have a 2004 4.8IS; had similar problems. My driver side would sag and my "self leveling suspension inactive" code would come up on my dash as well. I ended up spending $1000.00 to change the module because they said it was bad. The air bag suspension will do just what you are describing. Mine would go for a few days no problem then it would sag. I ended up replacing both front suspension air bags. Once one goes bad, the other side gets strained and it will be a matter of time before it goes as well. I took my X to Cosmo Motors in Hickory, NC. I know its far from Al; but they got me both front air suspension bags for about half of the price that the BWM dealerships were quoting me. And the bag itself was a rebuilt bag with a lifetime warrenty. Go on line and check out there website www.cosmomotorsonline.com the service Managers name is Mike. Hope this helps you out.