It is amazing that Lexus has not addressed the droning noise yet in 2006 models. Sorry but I think most of the GX470 trucks have this issue, but most people don't hear it, or don't go that fast 65-75mph.
I have a 2004 model that was purchased on December of 2003. Just after 2 days of owning the car, I heard the exact same noise that you are describing. After many trips to Lexus service, and many road tests I was able to establish a case which I took all the way to corporate level. Even I requested to return the truck back to them, but they claimed that this is normal sound in the truck and nothing can be done. The service foreman tried swaoing the drive shaft to see if it helps, but it didn't. So i can tell you that I did fight them, but didn't get anywhere. :cries:
So after owning the car for over 3 years now, I am kinda used to hearing the noise and it does not bother me as much. You can hear and feel it a lot more in winter time for some reason. Specifically when it is cold. It is just sad that they have not done anything to the design to fix this problem, even now?
Anyway, best of luck to you, and please let me know if you get anywhere with this issue.
Bob