In order to understand how an attack made upon you is dealt with or how your attacks on others are dealt with, one needs to understand the following:
For WHITE DAMAGE (not special attacks that give yellow damage):
1) Every attack performed by a mob on a player, or a player on a mob, must result in some "outcome" whether that be a block, crushing blow, parry, dodge, ordinary hit, etc.
2) These "outcomes" are determined (in theory) by a randomly generated number between 1 and 100 being compared to ranges of values on an Attack Table. For example: the game rolls a 45 when you attack, which falls into the Critical Hit range, so your attack is a Critical Hit.
Order for attacks received from a mob:
For Example: In order to push Crushing Blows off of the chart, Miss, Dodge, Parry, Block, and Critical Hit must account for all of the possible outcomes in your table, as these are the possible outcomes above Crushing Blow on the Attack Table. Note that Druids cannot Parry or Block.
Additionally, here is the table for players versus mobs and other players:
Druid Bear Tanks Want to Know: Can I become Uncrushable?
One needs a very high block rating to accomplish this. Druids cannot block at all, because we do not have shields. This leaves us with a chance of receiving a crushing blow, which is 150% damage. The Crushing Blow is mitigated by our armor. In theory, if your Miss and Dodge totaled to ~100% it would be unlikely that you would receive a Crushing Blow. This could be done with defensive skills. You would need to obtain a defense skill of appx. 2400 (50% Miss and 46% Dodge (assumes you have feral swiftness, if you dont +100 defense)). This is because the druid attack table looks like this when being attacked by a mob:
Basically, Uncrushability is for warriors, shaman and paladins- not druids. These classes have shields to block with and talents that allow them to Parry and Block.
So, for Druid tanking, have enough armor to absorb the Crushing Blows and enough defense to increase the probability of Dodges and Misses.
Note that Yellow Damage is likely reconciled with a two-roll system, which means that a Shred can be a blocked AND a crit or parried AND an ordinary hit. In this case + hit skills become much more important so that all of your special attacks hit the target to avoid a waste of energy/rage.
Thanks to the folks at these sites for my source data: