Time for me to chime in here since I had the same problem. It drove me nuts and I literally almost sold the car because of it. Every time I hit any kind of road imperfection, there would be loud squeaking and it was very audible standing outside the car as well as the cabin.
I changed to aftermarket LCA's, changed nothing (although this is a source of noise for many people and could be it.) Then I changed exhausts to a Pypes O/R H, and it got worse. So this lead me to believe it was the exhaust and it was. $2 later and the problem was solved. Check your exhaust hangers right by your mufflers (especially if you have an aftermarket exhaust.) Also check them by the transmission. The metal hanger rubs with the bracket and causes squeaking. Get some rubber, cut it, place it over the hanger near where it meets the bracket and zip-tie it. Problem solved.
Let us know how it goes.
'06 Satin Silver GT Premium