http://www.dke.org/conf.html
Getting through all the "legalese" basically it says you can join any off campus group you want. The school can choose not to recognize you or deny use of campus facilities, but they cannot prevent you from pledging any more than they could stop you from joining the Jaycees or the American Nazi Party.
Plus if you are at a public university, the school has even less leeway to discipline you for something like this. Our school (part of PA state system) blew smoke about expelling members of an underground fraternity on our campus but it NEVER happened.
DKE was very involved with fighting this, email their national HQ and they could probably get you some info on it (or just look through the news section of their website).