This last summer I moved from Atlanta, GA to northern AZ by way of Minnesota, SD, Wyoming, Montana, Idaho, Utah and Colorado.... I did this for my wife and her new Gov. job where they paied for the movine expenses if we kept records so I have certified CAT scale slips (see this post for specific totals : Linky ) Anyway, it was about 4000lbs and I had ZERO issues cruising at 80mph indicated, with the A/C on and on cruise... well, unless I hit a Really steep grade, then it'd drop to about 65mph at worst. Zero issues accelerating and the only 'bucking' type sensation was when I hit just the right distance between freeway expansion joints and then it'd get going a little bit (but was far from normal).

I have zero mods to the truck, and it was from 117k to 121k miles that I did the trip (or so).