If you do find an Envoy with the air suspension, the bags eventually develop leaks, and the air compressors go south (and are hard to find, as they've been long discontinued). That's a big reason why people swap out to the springs
(BTW, they weren't standard across trim levels on the Envoy - my '03 is a pretty loaded SLT, and it came with the springs). RPO is G67, from what I see ('suspension package, rear load-leveling')
I'll approach this from another direction - the seats.
An '03-'04 Envoy SLT will have dual lumbar in the drivers' seat (high / low position, although only one can be used at a time) They took it out in '05 and made it a single lumbar adjustment.
From what I've seen of people who've had / driven the short vs. long wheelbase 360/370's, consensus seemed to be that there wasn't *that* much difference in ride quality. And they'll share the same seats, within trim levels.
Although you indicated GMT full-size was too big - my '03 Sierra's lumbar adjustment also includes adjustable side bolsters (the 'wings' in the seat back). It's a leather-equipped SLT, as well. Good chance a similar-gen Tahoe / Yuke might have the side bolsters, too. Or maybe the dual lumbar - the SUVs tended to have a few more creature comforts than the pickups did.
Finally, if you don't find a replacement vehicle that works for you, you could always swap in a new engine (I see you have a 2WD 4.2, so if you could find a decent replacement engine from an '03-'05, you could keep the EXT.) Bonus with this is that it'll wind up being cheaper than a replacement vehicle, even when installation is thrown in. Depending on how many total miles on your truck, you could do a full engine / tranny swap, and be set for many more years.