That much brake fluid loss would trigger a brake light and you would notice a big difference in pedal feel. My guess is gear oil. You can always wedge a torque wrench or 2x4 between your brake pedal and seat (or have someone help you), and then go visually look at the brake caliper and hose for fluid loss.
More than likely it's the rear axle seal. Just did ours last week.
Yea thanks, I'm familiar with how insane it is to get them off
I actually just ended up buying Ten Factory chromos and I'll fix these up and have a spare set.