it'd help if you linked your armory then I can move this thread into the HALP! forum where people can give you personal critiques.
You may also want to read Satorri's guide to tanking with a DK in the Critical Guides forum.
to answer your specific questions
hit cap is 8% assuming you're using a single 2hander
dodge has no cap, it has diminishing returns, but there's no *cap*
same for parry
expertise caps at 26 for removal of dodges and something like 47 to remove all parries.
The situation most people are finding is because of DRs, you'll want to balance your dodge/parry ratings to be as close as possible to each other to maximize your avoidance gains while staving off heavy DRs.
One of my friends was playing his DK tank and our healer said the same thing - but he wasn't using Death Strike, was using abilities that he used as a Frost tank, and generally wasn't using any of his cooldowns effectively or at all - and it led to him being incredibly squishy. Make sure you read Satorri's guide, as Kazeyonoma pointed out, its a great help for new and returning DK tanks.
And the other thing I wanted to point out since Kazeyonoma mentioned it - don't dual wield as a DK tank. Two handers are the only viable way of tanking for Blood DKs. Not that you are, but in case you were wondering.
Some things about your armory link:
You have 4 unenchanted items, just make sure you get those enchanted up. Every little bit helps, so even if you have to use less than the most optimal chants, do so. You have mining as one of your professions, make sure you max it out to get the stamina boost (I can only assume that you were doing it for that, but it could be anything, maybe this is your mining character, IDK) and grab up a second profession.
I would recommend getting Blacksmithing or Jewelcrafting since you'll get the most benefit from having the ore on hand from your mining. Blacksmithing gives you access to the craftable epics, and the extra slots that blacksmiths can put on their gear, while Jewelcrafting will give you access to the very best gems for the most benefit towards your avoidance/stamina/mastery.
You also have a couple of dps items like the belt, the Neck and the rings. A 346 tank item would probably be better, although you have to compare them. Now you are missing on avoidance stats and you gain dps stats.
If you want to stay with them, than reforge the dps stat to avoidance. Keep avoidance and mastery.
Enchant your stuff, pick up a second profession and finish leveling mining - thats's a huge hp boost to start with.
Expertise softcap is still at 26
Hitcap still at 8%
But instead of maximising both i'd recommend aiming for more mastery and/or avoidance instead.
Posters mentioned it before, do you use your cooldowns generously? A lot are on 60s timer or shorter, no reason not to use them on every trashpack! Besides that, having a glance at Satorri's guide might help a lot.