Corpse tongue is a backwards Gylph of Indomitability that has the extra bonus effect of needing to be near death to do anything.
Gylph all the way.
Actually, it is not uncommon for a tank to be hit into the lower 30s. A single rotface hit can take a tank into those levels if he wasn't at full health when the biggie landed. The glyph is more controllable, but you can rely on an uptime capped only by the CD on the coin.
For now I would go with the Glyph of for all fights other than Gunship and maybe Marrowgar since Gunship favors avoidance and Marrowgar will usually get a good amount of hits on you while you are under 35%.