Only mobs 3 or more levels above your level can land crushing blows
Any creature 3 levels higher than you will land crushing blows
I did read that crushing blows by mobs less than 3 levels higher are possible if one's defense skill is less than one's levelx5. Ex. <300 at lvl60, <350 at lvl70.
There is some evidence to suggest that crushing blows can happen when the difference in the mob's weapon skill and player's (level x 5) is 15 or more, and is not entirely based on level difference as explained by Block. For example, level 35 mages and hunters with very underdeveloped defense scores taking crushing blows from same level mobs.
Crushing Blows do occur from attacks by creatures less than 3 levels higher than you, if your Defense skill is less than the maximum for your level (i.e. less than your level x 5). I have seen crushing blows landed on my level 36 hunter by a level 36 Highland Fleshstalker -- because my hunter spent most of his time shooting monsters at range and very little time being attacked in melee, his Defense skill was only 164, which is 16 points below the Defense maximum for level 36.
However, my defense is definitely maxed out at 350, and yet I was hit by lvl72 Hellfire Warders in Magtheridon's Lair with crushing blows. Another tank in my raid also reported being crushed by the Warders.
Did the Crushing Blow mechanics change, or am I on crack, or is my combat log mod bugged (I use EavesDrop)?