Had that happen to me the other day, is your AU
IRS? is the rear bouncy instead of the front? My rear upper shock mounts had eaten themselves and I had the same problem and after researching a lot of people had the same problem. Check behind the back seats where the shock mounts are bolted.
There is a member here (forgot his name) who modifies upper shock mounts that means you don't have to replace them. But yeah check your shock mounts first!