Whenever you have a vibration get your wheels balanced, if it still continues get your bushings in your steering rack, control arms, and other suspension components checked. When I had a vibration coming from my front right i took it to a local shop I trust and asked the mechinic what it could me, we jacked the car up and checked ball joints and etc by shaking the wheel left and right and up and down, when we moved to the right front we can clearly see the wheel moving and hear the tapping noise, looked at the steering rack and saw it moving and tapping onto the bracket, my bushings were shot. Not saying that its your rack bushings but just check over the suspension components piece by piece after you get the car balanced. If your suspension components and tires check out ok then you may have a problem with your driveline but you should feel it gradually get worse and worse the faster you go. as for motor mount and transmission mount youll feel vibrations as soon as you start the car and drive. I have had experience with shot tranny and motor mounts too -__-' usually you might as well replace all three so everything lines up well. If you do end up needing new items go try sewell lexus and use the Clublexus discount code, tranny and motor mounts should only come out to around 200 shipped.
98 GS400, slammed, drifted, daily'd