Utah to Georgia, best route?

I am driving from Utah to Georgia. I will be driving a '92 Ford F350 dually and pulling a Ford Focus on a car dolly. What is the best route? I70 through Colorado will be the hardest on the truck, so I don't think I want to go that way. I am torn between I80 through Wyoming and down through southern Utah, Southern Colorado and through New Mexico. Once you get past the rockies there are less mountains, but is the northern or southern route easier. My main concern is going easy on the truck. I am simply concerned that pulling the car up and down mountains will be hard on the tranny. Any recommendations? Let me know what you think.