Aluminum drive shaft kill a lot of the driveline vibration most Mustangs have. Plus it frees up a little power due to less weight(inertia) to rotate on take off.

Take the plug out of the side of the transmission and stick your finger in. You should be able to touch the fluid. If it looks like the stuff on the ground, then you know where it came from.

Could it have just been some dirty water from a puddle? Is the spot still on the driveway?

Anyway, the rear tranny seal isn't a killer. All you got to do is unbolt the driveshaft and pull it out. A little fluid always dribbles out anyway.