I was hired on as a salaried employee. I am scheduled for 40 hours but recently a colleague took a 2 month vacation and my manager has forced the rest of us to work additional hours to cover the open shifts. It actually is costing me to work due to my commute. Is this legal?

It is not illegal for an employer to ask you to work extra hours to cover for someone who is out, if that's your question. If your question is whether you should be paid for the extra hours, it depends on whether or not you are exempt from the overtime provisions of the Fair Labor Standards Act (FLSA).

Yes, you can be required to work add'l. hours if you are asked to. If by salaried you meant exempt, you can be required to work the add'l. time w/o any extra pay over your regular/fixed weekly salary.