Hey congratulations! I have a 98GS400 Millennium Silver. Incredible car all factory except the tires.
I had a similar problem, though my column wasn't shaking too much, just a strong vibration in the wheel itself, what I had done:
-Tire balance, recommended by Lexus dealer didn't work, made it almost worse
-Replaced the front tires, took 90% of the problem out.
-Changed the front rotors because i felt enough vibration braking at 60.
It's really a non issue now, but I keep my eyes, ears, feet and hands tuned in at all times because the car is that AWESOME and worth my attention
So I might recommend a second opinion on the front tires, in the end they were more banged up than thought on inspection by eye.
Tires wins races.
-Robert Duvall, Days of Thunder
Ha ha ha