@goofyfoot - what they usually do is pick a particular GD show from the past and play that set list. Of course, the performances are just as individual and improvisational as the GD. Sometimes they do shows though where they'll come up with their own setlist, but 'first set songs' are still in the first set and second set where they go, too.
Personally, I wish they wouldn't bother with replicating set lists - they needed that kind of gimmick when they first started, but they don't anymore - and would have them all be 'band's choice'. Lots of people like guessing what show it was though, too.
What DSO has that most of the 'other ones' don't (besides playing together for a few decades) is that Jeff Mattson not only plays like Jerry but sings just like him too. And Lisa Mackey is WAY better than I've ever heard Donna.
And it IS cool how they switch around lineup and standing positions depending on what year the show was that they're doing. So sometimes one drummer has to sit it out.