Druids can reach the armor cap against a boss-level enemy with current itemization:

Item	Armor
Pillar of Ferocity	550
Thunderheart Harness	3152
Thoriumweave Cloak	390
Umberhowl's Collar	281
Belt of Natural Power	423
Treads of the Den Mother	516
Violet Signet of the Great Protector	392
A'dal's Signet of Defense	367
Badge of Tenacity	308
Mark of Tyranny	180

This adds up to 6559 armor from items. The Druid's Dire Bear Form armor bonus, combined with the feral Thick Hide talent, gives a druid 550% of the base gear armor. This would result in 36075 armor, just over the 35880 needed to hit 75% reduction against a boss (This does not include any bonuses from Agility or enchants/armor kits).