You only get crushing blows from mobs 3 levels above you.  From what i can tell, the level cap for heroics is 72, and therefore it's theoretically impossible to get a crushing blow in a heroic instance.

OOOOH don't get technical with me!

Only your natural defense affects crushing blows (ie 350@lvl 70).  Buffing your defense to 490 will not help you avoid crushing blows.  What does affect crushing blows, is the ability of a warrior to shield block which adds +75% to his block chance.  As long as parry+dodge+block>100%, you can push crushing blows off the table. See the following link http://www.wowwiki.com/Crushing_Blow

What defense does do is reduce your chance to be critted.  490 is the commonly cited defense number to become crit immune.  Crits are similarly affected by a warriors ability to shield block.

Secondly, you're mistaking bosses in raid instances with bosses in standard instances.  You can be crushed by those and these are identified by your user interface as Boss ???.  You can tell what level normal instance bosses are by looking at your user interface.  A quick glance of boss mobs in normal instances has shown that the highest level for these mobs is 72+ and therefore crushing blow mechanics do not apply to these mobs.  Their crits in heroics will still hurt.

Edit: There are 2 things to consider when tanking, survivability which you can make up with gear, and threat generation.  Threat generation is the tricky bit with non protection tanking.  Like a poster above said, the defiance talent is almost a prerequisite for tanking considering the fact that your party member's damage will have scaled at a greater rate than a non prot warriors aggro tools = sunder, heroic strike and revenge.