From what I have been able to find it sounds like the defiant gear really starts to fall off lvl 70+. Where is a good place to start replacing this gear?
Find someone who is getting there orb for their epic 2.0 in citadal of anguish and tag along and get raid loot from that era
I used defiant till 82 then started moving up to the HoT drops. If you are a tank this may not suffice, but for everyone else I don't think gear made a big enough difference to worry about. Focus on leveling first.
I suggest a combination of the above suggestions and just whatever gear you find doing 70-80 content that you can do.