I'm also offended by the "Minnesota Vikings" because I'm of Scandinavian heritage and can be.
The "Dallas Cowboys" is a name derived from a historically pejorative term. Calling someone a "cowboy" in the frontier days was sort of like calling someone a "redneck" today. It wasn't a nice thing to call someone. In fact, where I live today, they still prefer the term "rancher" to "cowboy."
I'm offended by the name "Houston Texans," mostly because a team that sucks so badly shouldn't be out there representing the citizens of Texas.
"Oakland Raiders" is an offensive name because everyone knows that not all sailors are violent pirates (see "Vikings," above), plus the one-eyed man is offensive to handicapped people.
The "Cleveland Browns" is a name offensive to the citizens of Cleveland because no team should be named after a skidmark.
As for the "Packers" and the "49ers," I'm sure someone can think of a reason to be offended.
I’ve always referred to them as the 69ers