First off, there is no Babylon 6, and there never will be. It is explicitly stated throughout the series that Babylon 5 was the last of the Babylon stations.
As for what you saw on Crusade, that took place about 5 years after the second to last episode of Babylon 5, Objects at Rest. SiL took place 19 years after OaR, which means B5 wouldn't be destroyed until about 14 years after the first (and only) season of Crusade.
So, in short, Crusade takes place between the second to last ep of B5 and Sleeping in Light, the finale.
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Sheridan: Are you trying to cheer me up?
Ivanova: No sir, wouldn't dream of it.
Sheridan: Good, I hate being cheered up. It's depressing.
Ivanova: So in that case we're all going to die horrible, painful, lingering deaths.
Sheridan: Thank you, I feel so much better now.