2"x.120 wall hrew would be fine for the whole thing, I would probly mix a little 1.75 tube in there to keep the wieght down a bit, but the 2" definatly looks better in the fullsize trucks and offers plenty of strength. design is more important than material to a certain extent. Sounds like you are new to fabrication, make sure your welding skills are up to par before you glue this thing together, or have an experienced welder weld it up for you.
Good advice so far. For a fullsize rig that will weigh in 4k to 5k 2" x .120 wall HREW would be a min size. DOM is better if your budget can take it. Don't mess with CroMo your better off with DOM.I'd stay away from down sizing, just stay with one size and thickness. An expienced pro can make the call to where down sizing works, but many beginners will make mistakes. If all this means nothing to you stop right now and have somebody build it for you!!!
Your design and the fitment of the tubing will be the most important part of the process. I'd rather run a well designed poorly welded cage any day over a bad design with good welds. Welding is important but the lower your welding skills the more important copeing and fitment become. A good welder can safely weld gaps that average guys barely stick together. Most cage failures come from bad design, no matter how good the welds are a tube is only so strong.
Take your time and good luck :smokin: