I think of it as a small tank.

 

Mine wonders when there is alot of wind. I don't think that's out of the ordinary.

 

Between the "tire tracks" on the freeway and wind I'd say this is pretty normal. Anytime you don't fit in the "tracks" you're going to end up dancing left and right on the ridges .. All the vehicles I've had in the last 5-6 years have done it, you get used to it after a while.

 

One thing that you did not mention in your post was what you were driving before you bought the Hummer. As HummBob has so eloquently explained, it is basically a brick on wheels. And a brick that has a lot of surface area to catch any little gust of wind. Granted, the wide track of the aftermarket tires probably don't help any. But truth be told, its probably a combination of all the afore mentioned posts. Add that to being used to driving a more aerodynamic vehicle and voile, there you have it: A Hummer.

Enjoy!

 

also with a different size tire you may need your alignment tweeked i know my suburban with 37.5's needed more camber than the old tires.