Junior: No discussion of SDC or Branson. He just happened to be wearing that shirt.
Rube: OMG, I love 2 Broke Girls!
Tiffany: It was the newest one, I think; it was called "Anti-Social Network" or something like that.
***
Sabrina Rosen is the costume designer for It's Always Sunny. She's worked on three different Harrison Ford movies! Charlie Day, in addition to starring in the show, is a co-creator as well as co-writer.
Mike's right; the shirt was most likely randomly chosen at a high-end second-hand shop in L.A. The show is filmed in both L.A. and Philly, most likely the Philly shooting is just the outside establishing shots.