When Morris Moss, better known just as Moss, made a video for himself to use at an online dating service, he taped it while in his mother's bathroom. He explained his reason as being his mother's fascination with 'Diagnosis Murder', which she had the volume cranked up.
No mention of Dick Van Dyke or his character of Dr. Mark Sloan (a TV Crossover Hall of Fame member). So it doesn't have to be the actual show that we know from the Trueniverse. And with such a generic title, 'Diagnosis Murder' doesn't even have to be an American TV production. It could have been home-grown in the United Kingdom. Perhaps it's a show that deals with murder cases like all of those documentaries you see on Tru-TV.
Definitely not a Zonk!
'Who Wants To Be A Millionaire' isn't a Zonk either. It exists in the real world and Toobworld as well. Most game shows do. And if Chris Tarrant has appeared as himself in other fictional settings, then his membership in the League of Themselves can link those shows to 'The IT Crowd'.