I don't remember hearing about this. I do remember scattered reports of Republican offices being broken into though.
When angry Democrats attack - DemocRat Violence and Vandalism documents (Brownshirts for Kerry)
When angry Democrats attack - MN Republican Party website documents multiple incidents
Republican Party of Minnesota ^ | October 8, 2004 (updated?)
http://www.freerepublic.com/focus/news/1240786/posts?
Also some relevant threads far back here...
http://www.freerepublic.com/tag/electionviolence/index