A 265/70/17 wide tire seems a little on the wide side for OEM rims. That tire needs at least a 7" wide rim. It's probably bulging in the center of the tread, giving you the balloon effect. Not to mention, you have load range D or E tires on a half ton truck. I would expect it to bounce all over the place.
I used to run a Volvo VNL with Super Singles on the back. It would do like you describe your truck is doing, wander all over in the truck grooves. I was either driving on the shoulder or on the yellow line. Michelin X-ones solved that problem, because they flex more across the tread area.
I'm going to suggest a narrower, softer tire, and it will help immensly.
