My wife has been on my employers plan since 2009, after she got fired from her job. She was shortly added to my plan with no problem. We live in separate residences, however, we are still married. Never got divorced or even legally separated. My employer has dropped her from my plan(maybe because of negotiations with my union)out of nowhere. Is this legal?

What reason does the HR department give you as to why they dropped her? Be prepared to provide proof that you are lawfully married and not legally separated. Recently my employer required that all employees with spouses on their insurance provide a copy of their marriage certificate and the first page of the latest federal tax return (with numbers blanked out but showing filing status and signatures) to show that (a) they were lawfully married, and (b) they are still married.

Hello j:

I'd see your union shop steward.

excon

Yes, and even if dropped, she would ( in the US) be able to still be covered under the COBRA requirements.
But without notice and without being given a reason, no they can not and are not allowed just to drop her.