I tried to use J, but it's somewhat tied to my muscle memory already (and I'm surprised I use it quite often joining comments and stuff). If you're like me, remember that you can abbreviate :join to just :j.
The advantage of using J and K is that they're very vim movements, but if can't because of :join, another workaround would be to use <PageUp> and <PageDown> since they're redundantly bounded to <Ctrl-B> and <Ctrl-F>:
nnoremap <PageDown> 7j
nnoremap <PageUp> 7k
vnoremap <PageDown> 7j
vnoremap <PageUp> 7k