I would look first at the steering coupler. A worn steering coupler can cause drifting just as you described. I had a lot of play when turning and the vehicle drifted all over the road. I rebuilt the steering coupler and it reduced the drifting by about 85%. I believe that others have had the same result. You can buy a rebuit kit at the dealer for about $17. There is also a how to on this site about how to do the fix.
Good luck.
David
Good luck.
David