All,
I installed the Hummer Eibach springs that Stryder noted above, on my 2002 Avalanche, and very happy so far. Only a few pavement tests on asphalt with lots of speed bumps, and a short jaunt down a local four wheel drive road, thus far.
I made some measurements on my existing Z71 springs and the Eibach, to perhaps help others, as we have had a similar discussion in other threads, involving aftermarket springs, and non-Z71 applications. These springs will certainly fit Suburbans of similar year. My stock Z71 springs with a fair amount of unknown use, probably towing by previous owners, measured out at 9 coils and 17" of length. I have been running them with 1" spacers, for the last year. The Eibach's are 11 coils, with an overall length of 18.5". The coils themselves are 0.725" thick, as compared to the 0.710" of my stock Z71's.
I picked up about an inch of lift in the back, over my Z71's with 1" spacer configuration. For anyone shopping Hummer springs, you will find a left and right part number, from Chevy. These springs were not marked, and each rear side ended up at within a 1/4" of the other, at 39" from the ground to bottom of fender, on 265-70-17's. With my front lift keys on my Av, I have 2 inches of rake, which I think is about perfect.
As to compression, my Stock springs and spacers were pretty much junk, with my body weight bouncing on the tailgate or trailer sagging and bouncing considerably. With these springs, my 190 pounds on the end of the tailgate corner, maybe 1/4" of deflection. But the ride is absolutely not harsh. I am new to coil springs, but they seem to be designed well. I do need new shocks, as the Rancho 9000's I put in last year are either junk, due to trailering with worn springs, or just too light for my Av. Pretty sure my ride will get even better, with a better shock.
I did the airbags in mine, and that part of the installation went well. I installed the air lift 1000 bags. My stock z71 springs were 4.75 inches in ID, while the Eibach Hummer's are 5.25. The bags seem to be working, but do not have any trailering on them, yet.
I do think these springs deserve consideration if you have an Av or Burb that needs a little help. And Stryder, if you are ever up in Northern NM way, dinner is one me.
Craig