advice on driving CA-CO

I think I70 is pretty flat before Colorado. The road is jsut fine through Colorado with a "little" rise in elevation (about 7,000 ft) going through Vail. It is a lovely drive in which I have taken often and enjoyed.

I agree that I-70 would be a good way to go. I-40 is fine most of the way also, but when you go up I-25, there is one pretty steep twisty area through Raton pass that while doable, might be a little hard to pull in a U-Haul. Either way is a beautiful drive and well maintained roads.

I would choose I 70 also. Just remember you are driving a truck and you need to pay attention to all those signs you probably ignore in your car. When the sign says sharp turn 25 MPH speed limit, you need to slow down. When there is a sign with a truck tipping over, don't make the mistake of trying to copy it. Load your truck with the heavy items low and towards the front. Make sure you pack things tightly so there isn't room for anything to slip or shift while you are in the middle of a sharp turn. Take frequent breaks and talk with the rental agency about using lower gears during long downhill sections in the mountains. It's better to use a lower gear and keep your speed low than to try and stop because you are going too fast. Lastly, stay in the right lane as much as possible. Good luck.