Yep, like everyone said... tires. Also, has your car been in any accidents prior to this one? My 350z was in a right rear quarter panel accident and after I got it from the body shop I thought all was fine, but whenever I drove on the highway (especially in the rain) I always felt uneasy about the car spinning out. I thought it was because my car was lowered. Long story short, I went to get the car aligned and the mechanic told me I had a bent tension rod or camber arm (not sure which) and went back to the fu*king body shop and they fixed the issue that they FAILED to fix.
Problem solved.