Treasure Hunter will not boost your chance of getting a D Ring.

The "slot" in which Defending Ring can drop will ALWAYS "drop". It has an approximately 5% chance to be Defending Ring, 95% chance to be Pixie Earring. Treasure Hunter does not even come into the calculation, as that 'slot' is 100% every single time all the time guaranteed to 'drop'.

Treasure Hunter only affects things that do not have a 100% drop rate. Sometimes multiple items can be tied to the same "drop slot". In the case where two or more items occupy the same "slot", Treasure Hunter has no affect on which of those items in that "slot" will drop when the "slot" "drops". In this case, the "slot" in question will ALWAYS drop.

The sheer amount of misinformation and bad information about this game, especially at this point in its life, is staggering. Extensive testing has been done on all this. Visit ffxiclopedia or bgwiki and look up Treasure Hunter.