The Sierra Lean, had in on my 05' Sierra SLE, and I fixed it.
I had the same problem you guys have. It turns out the Sierra models for certain years have something wrong with the springs or spring assembly; I'm no expert, but I took mine to a local spring place, this is what buddy told me. To fix this problem, I bought new springs from a local coil spring company, who knew about the problem.
I looked up the info online here, to quote you. If you punch in the part number, SC20222 on almost any internet search engine, it'll link you to the answer you're looking for.
SPRING Part # SC20222 {Sold in Pairs}
Front; w/ AC; w/ A/T; Pickup; 2WD; Use to Minimize Lean If OEM Driver's Side Body sits 1/2" to 1" Lower Than Passenger Side
Order these and get them on the truck and your lean is GONE.
Cheers,
Steve