Some folks presume it's just a cost decision. And while cost is always a factor with GM, the more likely driver is because the engineers who invented the system did not want techs just replacing the ATF alone. The design calls for the filter to *also* be replaced every time at the same interval. The filter seal, or bushing, should also be inspected and is a _candidate_ for replacement as well if needed. Checking and cleaning the magnet is also called for. So forcing the pan removal just to drain the ATF helps drive _full service_ behavior. The _problem_ with this particular truck application, is the exhaust system obstructs performing this full service anyway. Making a routine maintenance job become significantly more labor and parts expensive, especially after a decade or more of corrosion on the exhaust system fasteners.
Yeah I went over that technique where you might get lucky, but only on later model years / body styles. Not happening on the generation this Avalanche is within.
I believe I showed this pages from the manual about this, 18-476 and 18-477. The engine does _not_ need to be running to perform the ATF hot check. You can leave it running, but not so long as to exceed the temperature range. You can leave it off, but not so long as to fall below the temperature range. The temperature range is what matters, the engine running is just a means to reach it. So use the engine to reach, and maintain, the temperature range. That's it. Hope that helps.
I'll give you the secret key. And this viedo is on RU-vid, but you unhook the muffler hangers from the back to the flex joint on the muffler pipe, put a 2 by 4 between the frame running under the muffler and rachet strap it down 2 inches. That's it, the pan will come out.
As I mentioned in the video, that approach (and other similar improvised ones) can result in damage to the exhaust system. Particularly if corrosion is advanced, or hangers are aged. So I don't recommend it.
