[fredjacksonsan]

I think tires can make a difference, even with the same size.

In doing my research to replace the Goodyear Wrangler ST's on my 2004 Grand Cherokee, the reviews in several places indicate that the wandering experienced on the highway went away when switching to Michelins LTX M/S.

I'm sure that a better quality tire can help, and agree with Rat's assessment that wider tires are more subject to the road conditions - after all, wider tires allow a greater moment of torque to be applied by a bump in the road, since it is applied further out from the center of rotation.