TankPoints is a mod that I use to do quick Crush Immunity calculations. It can also be used to compare gear. My issue with the way it compares gear, is that the algorithm is written from a very Warrior centric point of view. This makes perfect sense when you consider 80 to 90% of tanks are probably Warriors.
You can get the latest version of TankPoints here.
Once you get it installed, bring up your command interface in WoW and type "/tp calc"
This will bring up the TankPoints Calculator and it will look something like this.
What I use the Mod for is the Combat Table section. If you look at my combat table right now, you will see that I show a 15% chance for a hit to be a Crushing Blow.
You should also be able to see my buff bar mod. I do have Gift of the Wild (which helps your uncrushability) but I don't have Kings. This is usually the exact opposite of how I test myself. I usually test myself with Kings up but no Gift/Mark of the Wild because I can control whether or not I have Kings, but I can't control whether or not I'll have a Druid for Gift/Mark of the Wild.
If I was to be attacked by a Boss with the Combat Table in this configuration, you can see that there is a 15% chance that the hit would be a Crushing Blow, and that is a Bad Thing.
"But Honors, why do you tell everyone you are Uncrushable?"
Because I am.
Lets add my self buff of Holy Shield. Mikey likes this table mucho more. Not only has Crushing Blow gone to 0 percent, but so has Normal Hit. So if a Boss attacked me with the Combat Table in this configuration the Worse Case Scenario is a Blocked hit which would be a Normal Hit minus my Block Value. I'd still take damage, but I can't be Crushed, I can't be Crit, and I can't be Normal Hit. I'm Uncrushable with Holy Shield up.
Warriors work much the same way. They need their self buff of Shield Block up to be Uncrushable.
Shield Block and Holy Shield both work on Charges. Shield Block provides more Block (75% to 30%), but Holy Shield provides more Charges (8 over 10 seconds versus 2 over 5 seconds).
If all of the charges are consumed, the buff is lost and the table will revert to its non buffed configuration where the tank will be crushed.
Warriors have an easier time reaching uncrushable. Warriors need 27.5% mitigation (Miss, Parry, Dodge, Block) Uncrittable defense (490) alone provides almost one third of that. The Paladin has to come up with 67.5% mitigation.
Paladins are more reliably uncrushable. It is far more likely to receive 3 attacks in 5 seconds than it is to receive 9 attacks in 10 seconds. Some bosses even have mechanics that basically guarantee you will get attacked at least 3 times every 5 seconds (Prince Phase 2 is an example).