If I logged into wife's Facebook with the password and made a post to her friends that basically said she was nuts....can I be charged with a crime?

You misrepresented yourself, so yes it is fraud. If it is one time, and you’re not following her wherever she goes online, I don’t think it can be called stalking. But she is right for not taking it lightly.

They can also get your for unlawfully gaining access to the account.

What you did is illegal under Florida’s anti-bullying laws, which include the following behaviors”

“The definition of bullying includes a non-exclusive list of specific behaviors that constitute bullying, and specifies that bullying includes intentional efforts to harm one or more individuals, may be direct or indirect, is not limited to behaviors that cause physical harm, and may be verbal (including oral and written language) or non-verbal.” Stop Bullying Florida Laws

You should not have done it. I suggest you cease and desist and write her a letter of apology. Just hope that she won’t attempt to prosecute.

You need to apologize to her AND to all her friends.