First, armor does not affect your crittable status. The only stats is your Defense Skill, which can found on your character sheet and amongst your weapon skills when you press K.
Now, the confusing part is that there is a thing callged Defense
Skill and another called Defense
Rating which are not to be confused for one another. Defense Skill is what I've just talked about, and Defense Rating is what you can find on items.
What you need as a Feral Tank
with the talent Survival of the Fittest is 415 Defense
Skill. To reach that, you need 154 (or 156 according to some other sources) Defense
Rating from your gear. And yeah, the only thing that contribute to your Defense Skill is Defense Rating on gear.
For an in-depths explanation on why you need those amounts, you can check, and I strongly recommend doing so,
this WoW Wiki page. Basically that 415 is for level 73 bosses, and yes it does scale with level, but don't bother with that and just go with 415.
So yah, that makes you crit-immune. Crush-immune, on the other hand cannot be done as a Druid as far as I know, if you were wondering.