Boughta Sportage GS 2.0 T.Diesel in Feb 2010 and had exact same problem....lost the first set of tyres to poor tracking. I had the tracking done to get the last out of the tyres and have since had the tracking done every time I replace tyres. I have gone to same place for tyres as I want the tracking to be done on the same machine every time.Funny thing is though the last time i had four tyres replaced the guy told me the back two tyres were steering slightly left. When I told him he tracked the car the last time he was surprised. After new tyres were fitted I called back to tyre garage about two months later to show them the tyre wear. Both rear tyres are wearing more on the inside...front looks good. They advised that that is due to the car design rather than in-effective tracking methods.
I believe Lexus set up their cars to wear slightly more on the inside to give better road holding...if this is true then we have to rust the designer I guess.