Fresh 904....just stopped shifting.

My guess is the that the manual valve went out of adjustment
But wait. First;Go to the last paragraph,lol
My second guess is that roller clutch failed.
And third is that the pump drive failed, but it's only third because the other third would be that both clutchpacks simultaneously failed,hardly likely
And fourth is that something inside the rearend failed, like axle splines or spiders.

I would jack up the back and verify the rear end is good, then pressure test the reverse circuit, after verifying the manual valve was not between gears.
With no pressure, the TC hub is probably torn off.
With pressure, you'll have to drop the pan, the VB, and do an air-pressure test. If it passes that and no junk in the pan, then the driveshaft will have to come down, for to test the roller clutch lock-up. If that fails then, well, it's the first thing that went in there, sooooo the trans will have to come down.

Unless someone else has a better thought.

Hang on, what are the chances that the TC retaining bolts all let go? That's a simple inspection, so that's the first go-to for sure.If those are all secure, the back to the top.