They just need to give actual incentives for tanks to use the DF. A bit of gil is not going to entice anyone and that is really only for the high level dungeons.

Add tomestone bonuses for queuing as a tank in DF. ~10 Poetics and ~60 Soldiery for all low level dungeons and a ~5 Poetics and ~30 Soldiery bonus for all dungeons that normally reward those two tomestones. Obviously the bonuses would only be offered while there is a high demand for the job, just as they do currently with the gil bonus