First, thanks for stepping up and making personal sacrifices very few are willing to endure for their country.
As far as your physical disabilities go, welding is generally a physical career field. If your back knees, and shoulder are already broke down, welding may be a poor choice for you. There may be a few jobs that you could do, but your opportunities will be limited... perhaps aerospace bench work or something similar. I would investigate the jobs for which you are physically suitable before making any decisions and see if the opportunities and pay / benefits will be satisfactory to you.
As far as schooling goes, I recommend you thoroughly investigate the schools you are considering. They vary greatly in content, rigor, and industry reputation.
Good luck whatever you choose!