Thanks, Joseph. That short statement is very helpful.
Combined with Joseph's comment above, which looks accurate and definitive, here's what I "learned" scrolling around the web today. (Quotes because following may not be accurate.)
A commonly-used non-Z55 rear spring as Joseph mentioned above is a Moog 81069. But that, apparently, is a Heavy Duty spring and may give a higher rear end which you may not want, especially if not towing heavy loads. Some posters say that the Moog 81071 spring is lighter standard duty and may give a more stock-like street ride for non-towing use.
Commonly mentioned rear shocks are the Bilstein 4600 and 5100. They provide the exact same mechanism except the 5100 version is adjustable and not necessary if you're not fiddling with ride height. For stock-like ride-height, use the non-adjustable 4600. Looks like the model number for the rear shocks on a 2004 Yukon is Bilstein 24-143806
One post mentioned replacing the rubber coil spring insulator at the bottom of the springs. Moog makes model no. K160072.
One poster suggested installing airbags inside the coils, but this process is a bit too involved for me at this point, and I don't fully understand the purpose of such airbags. As I don't plan to tow anything heavy with my setup, I think a basic conversion is what I'm after. It looks like airbags can actually be inserted inside the spring without removing the spring, so I can think about that later.
One poster suggested getting passive rear air shocks from Arnott and leaving the original springs in place, but I'm having a lot of trouble chasing down my questions about that, so likely to bypass that process in favor of vanilla passive conversion.
Also, for other complete noobs like me, Z55 is one of about 115 three-digit codes on the "service parts identification" tag that specifies details about your vehicle build. That tag is located, in my case, inside the glove box. Z55 means the computer controlled air-assisted autoride suspension system. Folks also call these codes RPO codes for Regular Production Option. So the autoride suspension is RPO Z55.
