Holy Shit, Ron Swanson was on Deadwood by Alex Snider
Yep, Nick Offerman was in episode 2 of the first season as Tom Mason, the road ranger who killed Sophia's family and tried to kill Wild Bill.
Also, there's the part of the episode where you get to see his penis.
Gonna have to quote Ron himself on this one, "you had me at meat tornado". (I don't know what that could mean but it sounds like an apt innuendo. And disgusting.)