Another issue with our Aztek

Sometimes we notice something new that is happening with our Aztek,
as we are driving, i think when accelerating the steering wheel shakes something fierce. It happens sometimes just for a minute, or a few, and with my wife it just happened until she stopped at a stop sign, and started driving again. She said she got stuck in traffic on the "expressway" and when she got going again, that's when it happened. Any ideas what that could be?
I appreciate any help.
Thanks
Brian

This problem was obviously related to the caliper problem, and is no longer there.
Back running smoothly!