What can even cause it to leak this bad?
Well, the good news is that you'll find it if you pull the motor, so you may want to get parts ready. if you know the valley plate and intake and such are good, it's very probably the same issue I just had repaired at just shy of 300k.
On the back of the engine block, there is a cover that holds the rear main seal. The top of this cover also has an oil gallery that moves oil from the left to the right side of the block (for the heads, I think...). The cover has a rubbery elastomer seal that likes to go bad and cook, and when that fails it will spray oil. And it coats everything underneath... I had that issue for sure and thought my rear diff was leaking again at first. Usually cooks after about 150,000 miles, and it's on borrowed time after that causing a minor coating of oil that looks like it's from the oil filter adapter but isn't.
So good news, it's not the rear main. The bad news is that it might as well be.
Another option is leaks from the valley cover plate being loose, so you can check that, but probably not a common issue. It was loose on my truck, but I also was puking oil from the rear main cover plate. The valley plate was a gentle seep by comparison.