Re: Rear diff leak
Looks like the pinion seal to me too, but definitely check the breather tube and make sure it isn't blocked.
On edit: When the O-ring on my speed sensor failed, it made a mess just like your pic. I think the airflow while driving actually blows forward over the top of the pumpkin, then it gets slung around by the driveline. I thought for sure it was my pinion seal, but the axle shop discovered the bad o-ring.
2000 E450 dually V10 wagon