The bolt is off, finally! So I can resume work on the bearings and breaks. Oy.
I put liquid wrench on both ends of the bolt and let it soak for a few hours this evening. I don't know how much help that did, but I finally got it to budge by using the pressure plate as a lever to turn the bolt back and forth a couple of times, to free the threads. Then I was able to turn it out with a screwdriver.
Before turning:
Here's the bolt out with the presure plate:
Here's the end of the drive shaft:
The bearing assembly that you see here is the end of the drive shaft, which allows the wheel to go up and down which changes the effective length of the shaft, all while transmitting the rotating motion from the transmission to the rear wheels. There is such a joint at both ends of the driveshaft, called "constant velocity joints". The black goupy stuff is at least as greasy as it looks; it's a special constant-velocity grease just for this application. Judging by this joint, I would say that it's well lubricated and I don't necessarily need to worry about re-lubing them in the near future.
With luck, I'll be dis-assembling the rear axle assemblies in the near future.