IF you can, avoid dallas ft worth entirely by going down to San Antonio. Crossing Midland / Odessa going west was bleak and unpleasant, but not full of trucks a month ago, and winter hadn't really taken hold then.
I wouid avoid Flagstaff entirely in the winter. I can see it in the spring or fall on a return trip. I don't live out west, so maybe I'm being too cautious, but I was following a snow plow on a only one open lane, no turnaround road for 20 miles just north of Las Cruces in early January.