Ever since I purchased my 2008 QX56, I had a slightly loose Driver Seat. It didn't move very much it all, but i felt it every time I was braking a bit harder or accelerated. It was just a slight little clunk and movement in the entire seat. At first I suspected a worn out rubber bushing that rides on the electric motor adjustment screw for the forward and backwards movement of the seat. After all, the car has 183K on it.
So today I decided to take the entire seat out of the car and give it a good look over. I quickly discovered that it was in fact the entire screw shaft the motor turns in order to move the seat, that had a bit of back and forward play in it.
All I had to do was tighten that little Allen head screw in the middle of the face place of the motor. Be sure not to tighten it too much as that will lock up the motor from turning and burn it out. You want it to be as loose as possible without creating any excess slop. The best part, is that you don't even need to remove the seat in order to make this adjustment.