After much head scratching and throwing most of a ream of paper into the trash, I just found and solved the problem.
Thinking that perhaps I have a cable issue, I was re-seating the usb cable that connects my Xerox Phaser 3250 printer to the computer and realized that the printer was plugged into a USB 3.0 port on my computer. When I moved it to a USB 2.0 port, the printer immediately started working properly using the Xerox Phaser 3150 driver that comes with cups, just like it always did under CentOS 6.
The computer must have been pushing data to the printer too quickly over the USB 3.0 port; the printer got a garbled postscript instruction and everything went downhill from there.
Now I’m pretty happy — my printer works again. I’ve taped a note to the printer to be damn sure that I remember to always plug it into a USB 2.0 port in the future.