The J1 visa is a student visa (or so I've been told) and still does not give you the right to employment in the USA. It is meant only for the puposes of training and studying and not for employment. I to am preparing to go back to the USA on a J1 visa for the exact same intension as you, to carry out my CPL IR CFI CFII rating and hopefully gaining employment for a year as a CFI, I have spoken to a lot of people who have done this and have been employed there, but it is not 100% legal.
If you pay your taxes and keep out of trouble most people get away with it, but it's not something you should advertise.
I wont be when I do it anyway
LB