Transmission wo not Move, shifter works, transmission does not respond in any setting of the shifter?

Shifter linkage is good (doesn't move in park, can push in neutral), transmission doesn't respond in any setting of the shifter. Engine runs ok for a Ford (bank 1 lean, cylinder 5 misfire). This came on fairly suddenly, this shifted smoothly for 10k+ miles of mostly highway driving. Only a half dozen occasional not kicking in, a small rev and it would catch. Now, a few days ago after a short 6-mile drive, I stopped, put in park and idle for a few minutes then drive to next house to park. It acted like it was low on fluid, had to rev a bit to catch, same for reverse. Next day it is doing nothing, engine running ok, rev to 3,000 the transmission does nothing. I've physically checked the linkage, fluid was low, so I refilled, still no response. While trying to warm up transmission for properly checking fluid it doesn't warm up! Pan cold after 15 minutes of running engine. I'm guessing the pressure relief valve is stuck? My code reader doesn't pull trans codes and I don't have a trans pressure gauge set. I'm dumb enough to buy a Ford but smart enough to follow some directions, how can I assess this before pulling transmission? Thank you! :)

The vehicle listed above has an Auto 5R55S transmission.