My 06 had this issue from the day I bought it. You didn't even have to have that much weight in the back. All I needed were 2 adults in the back seat. I had AAL done to mine and that fixed this problem.

It created another problem though. From then on I had a vibration at speeds between 35 and 40 mph. Very irritating. When I took my X in for the mandatory rear differential replacement I mentioned this vibration and they said that it happens to some X's and that there was a fix that they woult order. They were axle spacers that were supposed to help line up the axle. Didn't really help. I suppose that whatever damage could be done had already occured. I still prefer the AAL and vibration to bottoming out everytime I came near a bump.