Rear Brake Vibration, 2010 Ultra w/ ABS

[MrSurly]

I'm trying to chase this down. I would be happy to get your input.

Upon moderate to heavy rear brake application I'm getting a pretty heavy vibration seemingly from the rear of the bike. It's been happening for a while and it doesn't feel like it's getting markedly worse or as if something's about to fall off. It feels like a warped rotor or metal to metal pad contact.
It's not ABS activation. I'm pulling it down tonight to inspect everything and I have new bearings in hand if needed.
At light application the rear brake is smooooooth. It's only after getting on it hard that the vibration occurs and it's quite consistent and linear. You can modulate pedal pressure and vary the vibration.
I'm fully expecting to find a crapped wheel bearing or a cracked rotor; some obvious 'smoking gun' would be nice.
Btw: clutch/throttle/ load/ gear/ lean angle/ doesn't make any difference.
I'll let ya know what I find but give me your best guesses.

EDIT: bike has 18,782 mi. Rear amer elite has 6300 mi.