My thinking is Glenn appeared on ABC around 2006 to 2007. But it could have been earlier. He joined HLN in 2006 with his first tv show. Not sure if that was the platform he used using to go on ABC or if ABC had him on before when he just did his radio show. But probably around 2006. Also not sure if he meant Good Morning America when Diane was there or when she started the Nightly News. I am guessing it was when Diane was on GMA and either Peter Jennings or Charlie Gibson did the nightly news.
Looking at Wikipedia, Sawyer didn’t switch to the nightly news until 2009. Beck was there with his TV show from Jan 2006 through October of 2008.
The thing that is interesting is that October of 2008 is when the media heads were threatened - but NOT ABC, according to the notes I took from Hagman’s interview with Laurie Roth. Fox, NBC, CBS, CNN, and 2 large radio networks were mentioned. ABC was conspicuously absent. Maybe because CAIR was already making sure they stayed quiet?