For 5 man dungeons even on my protection warrior I wasn't near to having +140 defense. I think it is just unnecessary as the any healer should be able to heal the incoming damage. There are other factors that are important as well. Like increased threat and avoiding downtime by having to drink a lot. I would also increase threat by adding hit and spelldamage to my gear. Increasing intellect and MP5 will help you to with mana management.
On my warrior I tanked dungeons most of the time when I was online. This is my character,
Dana on K1. Notice that I only have +44 defense on my gear and I never had anybody complain about being squishy or anything (I have a lot of HP though and two procs that will heal me often for a bit). I went for defense cap at first as well but I had problems with my threat and rage generation so I did rethink that approach.
So management of threat (spelldamage/hit) and your resource to use abilities (intellect/MP5) are equally important as improving your resilience (HP/armor/defense). When trying to reach defense cap with dungeon gear you will most likely miss out on threat, resources and even HP. It probably will make things harder for you.
Deathbone Guardian set will give you +62 defense. That alone is surely sufficient if your aim is only to tank dungeons. So I would build my gear around that one and add some of the other important stats I mentioned to it. Before you have the set pieces just replace them with similar gear that is easier to get. Getting that set can be tedious, but as protection paladin it is definitely worth it.
PS: I did tank most of the time as holy/protection paladin. Holy Shock is actually a pretty neat tool to have for tanking as you can use it as taunt and for ranged pulling. In my opinion for dungeons it has some advantages too and the playstyle can be fun. Perhaps in the long run the threat buildup is not as high but I had the feeling that I had way more control over fights, especially when I lost aggro with Holy Shock available. Divine Favor, Holy Shock and Judgement of Righteousness put a lot of threat on a single target almost instantly. I would not dismiss it as unsuitable for dungeon tanking that fast. I did not say it's better, it's just different.