I have just noticed the same issue in my 2005. I wanted to try to track it down but it is a constant, rythmic vibration. It is felt in the driver seat and it doesn't increase with speed. I feel it mostly in 5th and 6th gear when cruising. I have 144K and I haven't done motor/trans mounts yet or the drive disks.

I did just change tires, I do have spacers, I have KYB shocks and Eibach springs. It's time to do the mounts and the disks anyway, but I was concerned that it might be a more serious problem, perhaps with the transmission. Also, I was concerned that it might be some wheel bearings? Do we grease and repack these, or just replace?