I was under the car last night and noticed there is fluid leaking. It's a slow leak as there's only about a tennis ball size damp patch on the concrete and she's been sitting for 3 weeks. It's in front of the axle and under where the back seat is. It's a redish fluid which makes me think auto transmission fluid, but can't work out how it got there. The diff is relatively dry. I don't even have a garage (I work on it in the parking garage!) so it is really hard to get under and get a good look.

Any ideas where this would have come from?

 

Coming down from the back of the driver side rear seat? That is what it looks like in the picture. I would suspect a break (or overfilled) in your differential vent line. It is the rubber tube on the left side of your axle/differential and leads up behind the rear seat. But there should not be that much fluid in the line. There really shouldn't be any fluid in that line, it's a vent line that attaches to the filler hole (brake line block on top of the filler hole may make you think it is part of the brake line.)
Are the shocks up there as well (don't remember)... Is there any fluid that can be dripping from a busted shock?

 

Looks like your pinion seal is leaking to me.

 

Is that a drip on your brake line or is it a trick of the camera?

 

does it smell like rotting dinosaurs?

-Gun

 

If its red, it looks like you have a rear tranny seal that is leaking and splashing onto the brake line when the driveshaft spins and traveling down the brake line to the lowest point and then dripping.