I am also new to this forum. I also have a 2004 LX470 with about 180 miles on it.
I noticed the vibration on my first time out. I thought I felt something strange as is suggested in the other posts, but in all honesty, I had plenty of other things to keep me occupied during that first trip. I have been able to replicate the problem in other outings.
I am interested in what the root cause of this problem. It will eventually become an irritation.