I'm having problems gearing up as a newby LK Tank (level 70)

Some more stuff to consider:

Grab a friend and do the short quest to get your Dabiri's Enigma, in Outlands. It's a level 70 five-man quest, but I soloed it barely at 80. Ret will have a much easier time. Just bring one or two level 80 friends and you'll be fine. The pre-req is annoying to do as prot, so be prepared if you want to solo it.

Do your Wyrmrest dailies as much as possible. You want to have the cape at level 76, the chest and feet at level 78 for your dungeon runs.

Azjol-Nerub (72-74 dungeon) has a tanking ring drop. You'll want to read up on strategy before going in, else you will wipe on the first boss.

Don't forget blacksmiths can have extra sockets on their own gear quite cheaply, and common quality Northrend gems are cheap.

Two things, IMHO only:

Don't take Wolvar's advice and skip dungeons until you have the gear. Learn them while your levelling so you aren't clueless when you walk into a heroic the first time. Also, if you have rested XP, it's not bad gold/xp per hour, and certainly more fun than grinding alone.

I disagree w/ Maj on the mace, but can't fault his logic. I just think the Axe from Ragemane's quest wins, simply becuase every scrap of defense you can muster to get to 540 (okay 535) is gold. My axes were at skill level 80, and yeah levelling them sucked, but I got my Master at Arms achievement last night because of that axe.

The suggestion to go farm Dabiri's Enigma is a good one. When you want to go to heroics, you will have very few pieces with gem slots on them. ANYTHING that can give you defense skill is a godsend, and trinkets will be where you are lacking. There are no defense trinkets in WotLK while levelling.
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don't worry about not seeing any tank loot from quests early on as there really isn't any. wait until you're about 75 and in the middle questing in zul'drak that's when you'll see them start to come around. blizz probably figured that not enough people would spec prot until then for enough of a reason to not put in many tnak rewards. once you hit 78 you can equip the saronite set which will last you a while. a lot of people use them up through naxx because they can't afford the hit in defense until then.
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I find the ax a perfectly acceptable choice if you really feel you need every scrap of defense. I personally too the axe and even argued for it in the past. But what I found at 80 is I had enough defense with my daunting and saronite gear. You really only experience a defense crisis when switching to raid gear and 7 extra defense is not going to make or break you. It's less than half a gem difference.

I can understand people taking the axe but I can't approve of the sword.
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It is VERY easy to solo Dimensius for Dabiri's Enigma, you just have to think about how to do it.

Wear spell gear and stand in the doorway DO NOT GO IN. Heal the ethereal for as long as you can, tossing exorcism when you can. This will be harder for you because you are a Human, and don't have Arcane Torrent. Once the guy is dead, stand there and exorcism and Avengers Shield. You have divine plea, use it. Save Lay on hand until you are close to death and a shadow volley is INCOMING. Lay on Hands before it gets to you. Continue to kill him. It took me 30 minutes to kill him pre-wotlk.

For tanking, find a tailor and have them make you a Cloak of Eternity. Lots of Stam and extra Defense. This will last you until 75 when you can get the LW crafted cloak.

For a weapon.... that's a tough one. There are no good weapons in the beginning portions of WotLK for tanking. The first one you can get is at 74. If you "must" have a tanking weapon You could do SSO dailies for a blue quality at revered, but you will lose a ton of dps doing this.

You could put together a quick raid and go smash Kara in the face. You don't have to do too many bosses to get to the chess event, and you could get ahold of a King's Defender that way. (There is DK on my server who DW tanked with a King's Defender and a Sun Eater at low levels. That guys was INSANE)

The big thing is, looking at your gear, you are at 490 defense which means you are uncrittable by anyone in the nexus or Utgarde keep.

Easier ways to stay uncrit while leveling than finding a tanking weapon would be to get the cloak I mention, and get defense on shield and cloak. TBC enchanting mats are ridiculously cheap on my server, so the cloak should be easy.

Upgrade your gear with craftables as you can, run instances and do questlines for instances as these are how you will gear as a tank. Make sure to pay attention to reps. Wyrmrest, AC, and Kirin Tor are good reps as you level.

Since you don't have exalted with Scryers or Aldor, bust your butt on Sons of Hodir rep when you get there.
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Sunetra