Getting over the coast range and down to the Colorado River will test the truck on I-8. By the time you get to the AZ border on 8 you will know if your truck can pull up and down hill with no worries. You will not have any steeper or longer pulls than those enroute to Fort Collins, unless you get off of the interstate.
I would stay on 8 to Gila Bend and go north to I-10 there, follow I-10 to I-17 up to I-40 to I-25. There are a couple of by-passes around Phoenix between I-10 and I-17 (303 and 101) Maybe a Phoenix forum member will disagree but I would try the 303.
I-17 has a couple of long pulls and a 7 mile 6% down grade but that should not tax the truck if all is mechanically sound with the motor, brakes, and tranny.
Going north to I-40 and then over to I-25 will be a lot cooler and has more mountain scenery with some high desert too.
