With 146k, my 2011 QX56 w/HBMC gave my family a dangerous scare when the rear driver's hydraulic shock/strut blew its lower mounting bracket clear off the rear-front control arm. The control arm itself blew out like a jiffy pop.
In putting the truck on stands to check all of the suspension, a simple screwdriver test uncovered the passenger rear upper and lower-front control arms were easily compromised as well.
What caused all of these critical parts to rot and fail? RUST.
...but with less miles than two of my Suburbans, why did this truck rot out already? CHEAP components made out of thin stamped steel and welded together hollow with plenty of holes to allow the elements to sit and eat away the steel both from the outside in and inside out.
I'll attach pics.
With the control arms and the subframe in such poor shape, I elected to replace the whole rear suspension and subframe assembly. Rotor to rotor, diff, axles, everything.
The wrinkle: I didn't want to disconnect the HBMC lines and shocks/struts. In reading up, that was one system I didn't have the tools (pump and gage) for nor a dealer close by to calibrate it for me once back together.
The solution: I found a 2012 complete rear suspension assembly in FL and had it shipped to my house on a lift gate truck for $1800 total. Barely any rust in or out. It did not have HBMC though.
I put my QX on 12 ton/pair jack stands, removed the wheels, and went at the tearing it down. While I can get more specific on the steps if asked, below is the high level:
Disconnect the emergency brake lines in the middle of the truck where the left and right slide into the single cable bracket.
Unbolt the emergency brake lines from the body bottom.
Unbolt the drive shaft from the rear diff shaft
Unbolt the rear brake calipers and hang them off to the side.
Lower the spare tire and remove it. tie up the chain out of the way
Disconnect wiring harness from the air ride sensor (driver's side spring bucket/control arm), HBMC sensor (passenger side spring bucket/control arm), wheel speed sensors (both sides), and all along the subframe. move it out of the way
Cut (easiest) the upper control arms' (both sides) forward -position "leg" connected to the subframe. Unbolt and remove (leaving the rest of the upper control arm there).
* This will allow removal of the rear suspension assembly without disconnecting the HBMC hydraulic lines to the shocks.
Unbolt the HBMC hydraulic lines' brackets (both sides) from the subframe. Doing this from the top of the subframe while sitting up where the spare tire was is easiest.
Unbolt the shocks (both sides) from the lower-front control arms.
* This will obviously allow removal of the rear suspension assembly without disconnecting the HBMC shocks
Place and raise a floor jack to support the rear diff.
Unbolt the subframe (3 bolts on each side) from front to rear. The rear locating pins should keep it in place.
Lower the assembly to the floor
Drag it out from under the truck
Before jacking the newer rear suspension assembly up to bolt it to the subframe, remove both upper control arms by unbolting them from the subframe and pulling them (ball joints) out of the drive hub knuckles.
Since the assembly I got didn't have HBMC, I had to also remove the rear sway bar, end links, and mounts. no biggie.
Reassemble pretty much in reverse only leave connecting the upper control arms to last. You'll need to tilt and jigger the control arms around the HBMC shocks' hydraulic lines and then bolt them up.
To get HBMC to work, move the height sensor, brackets and linkage from the passenger side of the old rear suspension and bolt them up to the replacement.
A side benefit of installing the upper control arms last is you'll reverse the front top bolts so the nut is forward between the frame and subframe where before, you could never replace those arms without partially dropping the subframe to get the bolt out. Stupid design.
This is also a great time to replace the brakes and change the rear diff gear oil. Rechecking all of the bolt torques is also suggested. you never know what the replacement assembly had done to it before.