I was considering this as a possibility also after running the second toon up to 16. I confirmed Guarded Spin does stack for Tanks, with the same +15 DEF value/stack too. It's recharge is over double that of Divine Avalanche though at almost 7 seconds unenhanced. I don't have a TW Tank (yet, lol).
Can also now Confirm the +DEF buff from Divine Avalanche also does not stack for Tanks. Forget these swords, I'm going back to my Rad/Rad Tanker lol...