We had a yearly lease with a security deposit - later it went to a month to month and we will be moving out soon - since we do not have a regular lease are we able to use a portion of the security deposit against our last months rent.

No... legally you can not.

That security deposit is there for any damages over and above normal wear and tear AFTER you move out... and will be returned then if not needed, or any remnent abouve the damages... its NOT available for your use as rent before then.