Not normally a tank, but I got the Commendation of Kael, and I already was using one of the sta darkmoon cards alone with gnomeregan autoblocker. Should I swap out the darkmoon card for commendation, or autoblocker? I'm guessing the latter in most cases? :s
That depends on the rest of you're gear. For you to get some good advice about any gear question i think it's necessary to have an armory link to you're toon.
This specific question also depends on which darkmoon trinket you have.
Anyhow i will assume you have [item]Darkmoon Card: Vengeance[/item]
Then [item]Darkmoon Card: Vengeance[/item] + [item]Gnomeregan Auto-Blocker 600[/item] for threat
and [item]Darkmoon Card: Vengeance[/item] + [item]Commendation of Kael'thas[/item] for EH
Commendation + Autoblocker for avoidance (assuming you don't have any other trinkets to choose from). Neither trinket is really an avoidance trinket, but the commendation proc can help your avoidance periodically. Most avoidance fights are for fights that are a lot of weaker attacks, as opposed to slow big attacks, so increasing your block value by 59 (or 259 when you click it) will make a bigger difference than having more HP.
I suspect, though, that you have at least one blue trinket that gives defense, dodge or parry, though.