I've been experiencing a similar vibration for some time now. It occurred when I had the OEM shocks and springs, non XREAS. I have since replaced them with OME springs and shocks for a lift and leveling. Still going on.
I'm running the OEM limited 18" wheels and Bridgestone Dueler A/T REVO 2 in stock size. They've been road-forced balanced by Toyota, and my alignment has been done twice.
Typically happens at highway speeds, feels like my front wheel(s) have a slight flutter or are unbalanced. Really annoying. Front and rear brakes have been done in total as well. New calipers, the whole shebang. I have the Lexus driveshaft installed as well...
2007 V8 Limited, 133K.
My next steps will be to replace the intermediate steering shaft. After that, if nothing improves, I'm just living with it. I do recall some Lexus TSB about fixing a vibration from a transmission mount or something. I may go down that road if I get frustrated enough.