I think this is a myth.
For any team, you have 4 other teams in the division and 25 teams outside of it. So trades outside are 6 times more likely, but only based on quantity of teams and not on anything inherent to divisional play.
If pressed, I can think of young stars traded within a division. JT Realmuto, for example. Also Matt Kemp, Mike Piazza, Cliff Lee, Jose Canseco, Gary Carter, etc.