The 5100 Bilsteins are good but aren't designed as springs (duh). I'd say call Moog and order their version of the Z71 coil springs, which should be what you have on a 1/2 ton SUV. I can confirm that when I did my Z55 delete on my 2011 Suburban, the springs I got from Moog were about 250 lbs stiffer than the Z55 springs, They were also about 2" longer. That should help. Given what was done to the front end to level the truck, it's little wonder the rear squats with that kind of rake. I fight the same issue on my Suburban when towing as I "leveled" my truck by setting the front springs on the first perch from bottom on my front 5100s. I'm still working with my wesight distribution hitch setup to get things as level as possible with 500-ish pounds of hitch weight and water and an ice chest full of drinks in the back of the Suburban.
Good luck.
Edit: pretty sure springs on 2500 trucks are all leaf springs, springs on 1/2 tons, except the really older ones, are coils. Not sure how that works into your year but if 2500 leaf springs are an option, that could be a good move if they fit. Otherwise, Moog is your friend; call them direct and verify the part number for their stiffest coils. I don't think GM makes/stocks these anymore. I had no luck when I was looking for them.