GX 460 Drivetrain vibration

In have driven 10,000km. At around 5,000km it developed a mild vibration that can be felt mostly in the floor under the driver. It happens in any gear between 1,300 and 1,800 rpm. It is worse when it is cold outside and when the vehicle is laboring. Took it to dealer - still waiting for any reply from Lexus Canada (3wks). Otherwise an excellent vehicle. On a 6,000km trip along Oregon coast and back to Canada, I averaged 25 mpg (Imperial) with 4 adults and luggage.
Would appreciate any ideas.