I am new to lifting vehicles.
I started a thread on GMTNation
http://gmtnation.com/f76/replacing-air-springs-coil-springs-add-bit-lift-10273/ about replacing my air springs and doing a lift at the same time.
At this point, I am planning to go with the BDS kit, including the shocks
http://www.bds-suspension.ca/chevrolet-gmc/trailblazer-envoy/2002-2009/2-front-2-rear-kit/, and NAPA, Avalanche HD coil springs. They have the same specs as these Raybestos springs
http://www.rockauto.com/catalog/moreinfo.php?pk=2665844&cc=1440255. They say design load is 1300, and the spring rate is 229. That's a big jump over the OE replacements which have a design load of 815 and a rate of 159. I'm a little worried that the GMT truck would just be too light for those, but I just don't know. Maybe a good compromise would be XL/EXT springs?
I don't want the ride to be too harsh, and I'm not 100% sure that the NAPA springs are variable rate, although the Raybestos seem to be. Are the Avalanche Z71 springs variable rate? My concern is that the NAPA springs will be too stiff for the Envoy. I do tow a 3300lb (dry) travel trailer a couple times a year. The other option I was considering are Envoy XL springs in the back.
The off-roading I do is mainly limited to potholed logging roads, and river banks with large cobbles. Last year I just scraped the oil drain plug on the river bank.
I am running LT245/70/R17 Firestone Transforce A/T tires. More highway tread than A/T, but they work for the stuff I do.
EDIT: I can't seem to figure out what my front springs are. RPO codes appear to be 6SP and 7SN. I replaced my struts a few years ago, and I think I removed the tags, I'll have to look more closely next time I have the tires off.