I had this exact situation come up in a friendly game this past Saturday. A unit of my Slaves exploded after losing combat to my friends Hammerers and he asked me, "So D3 + ranks S3 hits, as in full ranks or just ranks?" I cocked an eyebrow and couldn't answer. I always assumed it was full ranks but the Cornered Rat rules simply mention ranks. We played it as ranks (full or not) but it didn't feel right... especially when the exploding slaves robbed my nearby Doomwheel of its last wound! Given a Slave unit's cheap cost and Vetock's numerous fumblings in the Skaven book I get the feeling he meant full ranks. Since the Cornered Rat S3 hits are automatic it seems silly (and unfair) to grant such an inexpensive unit that kind of killing power post-mortem.
Then again... maybe this is Vetock's way of letting the abused and downtrodden get their revenge on a cruel world...