Front Suspension problems of the 2002 Nissan Quest

We purchased a 2002 Nissan Quest van in February, 2003 from asheboro, nc Nissan. The day that we purchased the van, we experienced an oil leak. This problem was repaired by dealership, and we have had no other problems. In March, 2003, we started experiencing problems with a vibration in the steering, console, seats and floor. Asheboro Nissan replaced one continental tire in April then 2 more were replaced in October (same tire make replaced). Since all of this, the asheboro Nissan dealership has backed out of our problem, and I am now dealing with kenneth carter at Nissan America in la. We have now had a total of 7 continental tires on our van, and the problem still persists. My wife was told by jeff at asheboro Nissan months ago that it may be a problem with the axle. I am not sure if this was ever checked out. I was told to file this complaint by the nc atty. Generals office.