I just had my GS 300 in the shop today, and the mechanic looked it over on the lift and we even test drove it. He told me that everything was normal and my car was getting old. I have a 1999 with 52K miles, damn I hope not. Anyway, i have heard reports of the ball joints going out or even a bearing.
After everything, I put a new version of the factory tire on, and its still having the problem. I try not to think about it when i am in the car, but its damn hard! Best of Luck! i will post again if I find out some new info. Make sure you post if you find something out too.