I have set my campaign a couple years before A New Hope, but we are steadily marching towards it. My characters know they are in an alternate universe, but they haven't found out what is different about it yet.
The thing that's different in this universe is that Obi-Wan Kenobi was killed the young Darth Vader at Mustafar.
Effectively I've created a scenario where when the Rebellion is at their darkest times, the heroes of the Rebellion won't be around to save the day. In fact they may very well be enemies.