Postdoc without PhD? I have never been to grad school, but I have several publications, in good journals, as well as planned future projects. Some very prominent experts know me and think well of my work. In this case, should I apply for postdoc or just go to grad school?

If it matters, I am working in a subfield of mathematics.
At this point in time, a Ph.D. is generally a non-negotiable requirement for a postdoc position.  If you are already carrying out independent research, however, then it might be possible for you to obtain a Ph.D. in graduate school in a significantly shorter period of time than normal.