What's wrong with a little over an inch of sag? There could be more options but the only 2 I see are get a weight distribution hitch or stiffer rear springs. Stiffer springs will obviously make it ride harsher when unloaded.
Personally I wouldn't worry about it unless you're always towing.
I do tow my boat a lot 9 months of the year to fish. In my original post, I remarked my Tahoe suspension seemed too soft and asked for recommendations on stiffening it up. To me, the fact it drops over an inch when connected to trailer is a symptom of a soft suspension.
Since my original post, I've not found anything that will work with the IRS. I do not want a weight distribution hitch nor do I think it should be necessary for an suv with over 8,000lbs towing capacity that's only towing a mere 3400lbs.
It's my bad that I did not research enough and bought one without air suspension.