Crushing Blows are the lowest part of the attack table, and are only available to enemies that are a certain level higher than you, 3 or higher.
In TBC the formula was 102.4% avoidance. This was achieve via 5% from miss chance inherent to crit immunity defense cap, and a combination of your block, dodge, and parry.
Essentially once you have a 102.4% avoidance stat, you can ONLY mitigate an attack. You cannot be hit by any crits, crushing blows, or normal attacks.
Warriors in vanilla have 100% block chance with shield block. Combined with their default 5% parry and 5% dodge, warriors are naturally uncrushable.
It is virtually impossible for druids to ever reach this, in both TBC and Vanilla, since they can only dodge. They must just eat a crushing blow, using their higher health and armor to mitigate more of the damage.
Paladins can theoretically reach uncrushable depending on how gearing works.
They can get a 62% block chance with Holy Shield, Redoubt, and the Holy Shield Libram. With 5% base dodge and parry, they would need 30.4% additional dodge, block, or parry. Potentially 25.4% if there is a default 5% miss chance.
It is unlikely at this time that paladins will be able to get that much combined avoidance on gear, even with the defense increases. But it is much more likely than druids.
Rogues can reach 100% dodge with their cooldowns, and have a lot of increases to parry. It is possible they might also be able to reach uncrushable temporarily, but it is almost certain that if they can reach uncrushable passively it will be from parry+ dodge which will be nerfed, as it means that a rogue can never take a melee attack from the front unless stunned.