Any update on how this is riding?
How are the Moog coils working out? Did you go OEM with your control arms replacements?
I recently ran into this same coil spring issue (2011 Denali 6.2l AWD). My RPO codes are Z55 / 6A5 / 7A5
The p/n for the top nut was listed as 15815519 but I’m guessing they might be different than yours since we have different codes.
I am quite happy with the ride, and it is much better that the cheap aftermarket non-Z55 suspension parts that the previous owner or seller installed. The Moog springs are working fine, so far. I did go with genuine GM upper and lower control arms, which included the ball joints.
For your 2011's RPO codes that you posted, Rock Auto has front coils listed with a part number 20842473 at $69.79 each. I cross-referenced this part number on GMPartsGiant.com, and their cost is $85.24. HOWEVER, when I cross-referenced the part number on GMPartsDirect.com, it stated that this was not the correct part number, but instead, the correct part number is 25783732, at a cost of $58.76.
As such, you have a discrepancy here regarding the correct GM part number for the front coil springs that should be resolved, prior to you making any purchases. Please post the last 8 digits of your VIN, and
@915_Tahoe can tell you what the correct GM OEM part number is for front coil springs is for your specific truck. He can also tell you what the correct OEM part number is for the nut for the top of the OEM strut/shock shaft.