Hate to break it to you, but you are very, very wrong about what stats are important to a tank. The most important stat is defense until you hit the cap (which depends mostly on the level difference between you and the boss), after which defense is -still- a good stat. Mostly because anything with defense? Still gonna have stamina and strength, almost universally (there are a few exceptions, but by and far, anything with defense, has those two).
Defense is the most important stat until you hit the crit cap. Unfortunately, as you're levelling it's hard to measure what the exact crit cap is since defense diminishes in value as you level, and the crit cap is dependant on the difference in levels between you and the boss.
Generally, crit cap is [Your level] x 5 + 5 x [Difference in levels between boss and you] + 125. So at level 73, if you were fighting a level 76 boss, your defense cap would be 505. Anything less, and the boss can crit you.
If you have defense gear, it generally comes with some dodge/parry/stamina/strength/etc, anyhow, so primarily you're hunting for anything that has defense on it, not just that it has stamina.
Strength is, in fact, one of your least important stats. Yes, it helps your threat. It also vaguely helps your parry as a DK. No, it's still not a stat you want to even worry about unless you're finding you're having threat issues. Even then, I'd probably tell you to look at hit rating and expertise first, given you have very little of either.
Similarly, agility is again not that important. Mostly, it's good because it vaguely helps your dodge, and ups your crits.
As a tank, you really have a lot of stats to look at. Strength and Agility, however, are not even -close- to your main stats.
Some stuff you definitely want is Tempered Saronite X, from blacksmithing, at least once you hit level 74 (as I believe that's required to use it). Alternatively, search the AH for green gear with the suffix "of the Champion", which is generally a mix of stamina, strength and defense or the suffix "of Defense", which is just defense, but generally a looooot of it. Or just look up wowhead for blue gear with defense that you can get through questing or instances, and hunt for it. Or look at those Cobalt X items that Bjorn also mentioned, although those are greens.
Go browse through
http://www.tankspot.com/ for a lot of information on tanking.
Bjorn's right, you're geared mostly as dps right now, not a tank. This is actually still alright for normal instances to some degree, you're just working your healer tree friend really hard by not being crit immune, but it's still doable.
Once you hit 80 and try to do heroics, however, your friend is REALLY going to hate your ass while it's not def-capped, because you won't just be working him hard, you're going to stress him into a heart attack, assuming you guys somehow stay alive through those heroics. And that's assuming people don't just look at your gear, go "Ewwwww" and jump ship on you. And forget about even attempting raids without def-cap.
Oh, and Bjorn is right about the weapon speed. You really want slow weapons when dual-wielding. This is because a lot of your damage depends on the damage range of weapons, not the dps (Frost Strike, Obliterate, Blood Strike, etc). In your armory page, you have a coldstone cutlass (2.6 speed and 287 max damage) on your off-hand and a stoneblade slicer (1.7 speed and 187 max damage) on your main hand.
You really want to get rid of that stoneblade slicer. Baring that, however, you should at the very least switch the two weapons, so the cutlass is in your mainhand and the slicer in your offhand.