I tried searching but no luck. When I go over 80 mph, the steering wheel becomes completely loose and has alot of play. With this comes alot of vibration. I've been through 3 sets of rims/tires and have had the problem with all three. So its nothing pertaining to tires. My car has almost 235,000 miles and as far as I know its on stock suspension. When I drive lower than 80 its fine, no vibration, no loose feeling. Could there be something wrong in the steering column?

 

have you ever changed your control arm bushings?
my friend has the same symptoms on his sc with 250xxx miles..
changed his bushings and all is good.

 

my 2 guesses are either wheel alignment or something with power steering.

 

You also might want to try replacing the steering rack bushes - I had similar symptoms and now the steering feels so much tighter and I feel in control of the car again. FYI, I got the steering bushes from Vlamos in Australia - about $45US I believe. GREAT UPGRADE. Also, go ahead and do the control arm bushes, only need to do front lower bushes as these seem to be the ones causing the majority of problem. You have options on CA bushes, Vlamos sells, and now u can get stock Lexus as well... hope this helps.