I don’t remember much about the plot. A large part of it takes place in Sam’s office where he’s arguing with either Diane or Rebecca. Every time the door to the office opens you hear part of an (increasingly heated) argument between the regulars at the bar, ending with one of the regulars shouting, “Oh! And I suppose next your gonna say that Wile E Coyote is the Antichrist!” or something like that.
I tried searching (both the SDMB and Google), and came up with plenty of evidence that what you’re thinking of exists (e.g. Post #11 of Favorite ‘Cheers’ moments), but alas, no indication of which episode it was.