defense is for crit mitigation.
armor is for damage reduction.

just load up on armor since no mob will be able to land a critting blow on you... unless, well... you meet a hordie.


if you want your pally to boost your alts in lower instances, that's probably all you're after. buy a bunch of level 69-70 gear. do some of the world group quests or take him through some instances. check the database site of your choice for instance drops, quest rewards, etc.

consider pvp gear as well. plus the starter pvp sets when you hit honored with some select factions (CE, HH, Shatar, Lower City, Keepers of Time).
tanking ring from final boss' chest in Ramps, tanking ring from Epoc Hunter in Old Hillsbrad, Cloak of Valliant Defender, Ogre slayer band, crafted Necklace of the deep... these all come to mind and are pretty easy to get. rest of the gear you'll have to figure out. hehe. but you can boost lower level folks up with those gear, plus some blue crafted gear from the AH. not too hard. those items are entry level 70 instances, heroics, and kara gear anyway.