Hmm...don't know if I'll be any help or not, since I have a 4.4i with 2 rear air-suspension not a 4.8is with 4.

But, the message pops up on mine quite frequently and the dealers still haven't figure out what's wrong after more than a "YEAR" on trying to solve the issue. They'd changed a new cluster, upgrading the computer, new control module, suspension height sensors...etc etc. Stuff you can think of that relates to the suspensions. But still with no solution.

I only just went back again last Friday about this...problem. And, they found that the height of the car was out by 20mm on one side. Like the car's tilted itself. I think they're going to re-new the pumps for the air-suspensions next and see how it goes.