Im a pipe welder by trade, and our standard is 6010 root pass for penetration, and a 7018 cap. 6010 and 6011 are good rods for penetration, but is much more brittle and apt to crack. If the weld you're making is taking any sort of torsional abuse, you should use 7018. In my honest opinion though, on an axle application, running a stick machine is too much heat anyways, and if youre not a seasoned welder, you can bend your axle tubes. I would use wire on good clean metal, and make sure you get good penetration. More is not always better.