We are flying into Munich and then driving to Italy. Are vaccinated U.S. citizens allowed to drive through without getting a travel certificate? Any other information regarding driving across borders?
According to many websites regarding Austria and the lockdown, you are allowed to transit through Austria without any stops. You might want to check to see what Italy says about travellers arriving from Austria.
Some information addressing your specific question (transit only journeys) is provided on the following page at Austria.info. Scroll to the second drop down question from the bottom of FAQs. Note that proof of destination is required. It also states that hotels, restaurants (except take away), and other facilities that cater to tourism are closed. Refueling stops and toilet use are permissible. Current covid details are available at Austria.org.
Stay safe, be well.