If a picture tells a thousand words, how many words does a video tell? Turn up your speakers so you can hear the air compressor going, and the eventual "pop" on the second video.
Here is the shock absorber (left rear), seconds after key-on, descriptions are below each video.
You can see that the compressor comes on and "partially" fills the shock absorber, as you can see it rising, as it almost hides the upper shaft. But it doesn't go all the way to the top. This "gap" is what makes the noise when going over bumps.
Immediately following the initial key-on, I keyed-off, then keyed-on again, commanding the compressor to pump pressure again. You can see in the video below, how the additional air pressure fills the shock absorber even more.
Notice that there is a loud "pop" where the shock absorber fully fills with air (thanks to a second initial compressor pressurization), and it strikes the built-in stop at the top. According to the manufacturer, Gabriel, they should always be all the way to the top. Note that this pressurization that happens at startup does NOT affect ride height, because the 00-05 Deville/DHS is level when empty, without any air-lift assist. It is only when it has additional weight that the ride-height sensor, notices the lowered rear, and commands the compressor to run until it detects it is at level.
CONCLUSION.
If OE is preferred, they don't make the original part anymore. But if the AC Delco branding matters go with the 504-547 and replace the dryer spring, lines etc that come with the kit. You'll have to purchase rear shock-absorber mounts if yours are worn (unfortunately I discarded my old shocks and threw away the mounts with it, because I purchased the 504-123, else I would've probably reused them). You can also get the Gabriel HiJackers (exactly the same kit and air-lift shock absorber, for cheaper), or the Monroes which require no modification at all, and do not have this extra shaft at the top that makes the noises (they cost even less than the Gabriels).
If you get the 504-123 you will have the same shock absorber with a direct-fit air line connector (as the Monroes!), BUT it won't have the spring that goes inside the dryer. You may have the noises coming from the trunk and the useful life of it may be shortened.