Differences in termination decision

Illinois.
I said something inappropriate at work and employer didn't wish to keep me on. My boss offered to let me " Voluntarily resign " to save my profit sharing and to allow that on future applications. I received a letter saying I have been terminated for "misconduct" and that I may not continue health benefits.
Can they do this, and is it O.K. if they tell prospective employers that I was involuntarily termintated?

Taking the prospective employers first, yes, they may. A forced resignation is the same as a firing in most if not all states. They are allowed to say anything that is true or that they have reason to believe is true when a prospective employer calls. Just because your boss decided to allow you to keep your profit sharing does not obligate upper management to lie to prospective employers.

As far as whether they can deny your health benefits, it's difficult to say without knowing what it was that you said. Under the law, they are required to allow you to continue your health benefits at your own expense in all cases EXCEPT gross misconduct. That's GROSS misconduct, not just misconduct. What is considered gross misconduct is not defined in the statute but in cases that have been filed with the DOL it has been translated very narrowly.

So if you told your supervisor to f--- off, they can fire you and they can tell other employers why, but they cannot deny your health benefits. On the other hand, if you made a death threat, that might well be considered gross misconduct that would be worthy of withholding COBRA.