One of the quickest ways to help your defense and overall gear/gem selection is to replace that Keystone Great Ring, you aren't a Druid so it is really limiting you with your current gear.
If you are fairly rich you could simply grab a Signet of the Accord - Item - World of Warcraft from the AH but if not it is fairly quick and painless to grab a Unsmashable Heavy Band - Item - World of Warcraft from Utgarde Pinnacle
That will make your life much easier with regard to staying over the 540 required def cap. Once that is taken care of you should be ok to move on and just wait on better gear drops from Nax. Try to never gem for defense if at all possible, it isn't worth the loss of TPS that exertise & expertise/stam gems provide