It's a nice weapon, the King's Defender, but by the time your capable of reaching it, you should be close if not over defense cap as well as the armor cap. (hence why the +spell damage seems the best choice) That being said, while it may be lacking stats in line with adding to your defensive ability (which your not gaining anything of with the current gavel), the weapon seems to shine in t's speed. 1.6 is fast and the dps is significantly higher than your current weapon. This speed should help a bit more with aggro retention when combined with your other abilities and righteousness should be cranking out alot more damage than what you're doing currently.

Perhaps as an alternative, toss the 40 spell damage on it, replace another piece of your tanking gear with a similar +defense stat with a spell damage item and then enchant that item as well. I guess it really comes down to your current spell damage and how close to defense caps you are. You're going to want to push close to 200 spell damage as you can whilst still maintaining your defensive stats.

Hope that helps!
Max