I was terminated with just cause and would like to know, legally what can an employer tell a future employer about reasons for termination?

Anything that is true, or their opinion of what is true.

Which basically means they can say anything, because anything can be someone's opinion...

Well, there are outright mis-statements of fact. "Joe was fired because he had a bad attitude" is an opinion. "Joe was fired for stealing" is a statement of fact; if Joe was NOT fired for stealing, then they cannot say so without possible recourse by Joe.