No, Druid Animal Companions don't grow by HD advancement. The table in the Druid entry just add HD (and other benefits), but those aren't monster advancement HD, just HD from being a Druid's Animal Companion. (See [url="http://www.wizards.com/default.asp?x=dnd/rg/20070206a"]here[/url].)
Shapechange (9th lvl) lets you turn into creatures that are immune to criticals.
When the four Heart of XYZ spells from Complete Mage are active, you're immune to extra damage from Criticals & Sneak Attack. (They last hour/lvl, so it's quite easy to do.)
Druid Wildshape, while not a spell, can turn you into an Elemental later on, which are immune to criticals.
If you're talking about the Divine Shield feat (in your title), yes, that one stacks with a Monk's AC. The ring however, does not.
BTW, I assume you know of the Sacred Fist PrC?
