One of the most common questions Tanks of all classes ask is "Is my gear ready for _____".
I completely understand the question, especially as an offspec tank. Tanking is a very visible part of any encounter. In many encounters, when the tank dies, it's game over.
Tanking is in many ways about confidence. Is your group confident in you? Are you confident in yourself?
Most tanks fight a fear that if they go into an encounter undergeared, they will wipe the raid and lose their opportunity to tank.
You've got to realize, it's not about you.
Just get a group together and start killing trash. Try the boss, see where you and your team needs to improve.
If you are dying and the healers have mana, you need more Stamina, Armor, Block Value. If you and dying because your healers ran out of mana, more dodge and parry would be a good thing.
Remember too, it's not about you. If you have the DPS to take Gruul down in 9 growths, you can tank him in very different gear than a group that has DPS that let him get to 16 growths.
Last point. There aren't a whole lot of fights where it's just a matter of sitting there and whacking away at the boss while the healers heal you.
There is nothing about your play or your gear that is going to change 3 healers getting owned by a Shatter, or a Hunter using Arcane Shot on Krosh, or people not moving in and out for Eagle Boss' Storm. It's not about YOU, it's about the team.