I have seen with only 2 points apart on the PD once so i do believe it must have happened at least once. There are a lot of bot leagues in this game (1024 leagues in every D.VI for example) so it must have happened at least once.
I missed the playoffs one season by two points of PD - and that two point difference was a buzzerbeater dunk by my opponent who was already in garbage time.
(http://www.buzzerbeater.com/league/10/overview.aspx?seaso...) (http://www.buzzerbeater.com/match/42762325/reportmatch.as...)I imagine there must have been some PD ties. I think the chances of a tie in Points Scored and Allowed are very low, though, but the thing with very low probabilities is that eventually it'll happen once somewhere if given enough opportunities. (Which, incidentally, is the title of Shaq's Free Throw Shooting instructional video)