You know, I struggled with the "tail dragging" issues on my '04 Suburban too. Tried air bags but they wouldn't stay inflated (not sure what I did wrong - might have to try reinstalling?) but what finally worked was a weight distributing hitch (WDH) on the trailer.
I don't know if that's an option for you but they seem to work great. Yes, they're heavy as hell and the take a bit of time and tweaking to set up but they completely eliminate the rear sag and as a bonus the one I got (Fastway E2 round bar system) also acts as a sway control. And unlike the type that uses chains, I can back up with this one.
Here's the link:
https://www.amazon.com/gp/product/B0071L3H2E/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1
If you've tried other methods and they haven't worked it may be worth it to try a WDH.
Couple of nice things about the WDH:
1. Requires NO modifications to your vehicle so you won't have to live with a too-stiff ride when you aren't towing, and
2. Completely portable and removable - if you get a new trailer or a new truck, you can use the WDH system on the new trailer and truck as well.