Here's an update to this topic.
After a year of hard work on the coupler fix, it has happened again. Now that I know what the sound of a spinning coupler is, I ordered another coupler and and pin, so I could repair it myself.
I took it out of 4wd immediately after hearing the sound, so I was pretty surprised to see that the splines on the pinion are almost non-existent. The splines on the inside of the coupler were worn down, apparently to the point that they couldn't hold onto what was left of the pinion splines. The pin was still in place, but had split into two pieces lengthwise. After actually working on this myself, I can see that the pin is not a shear pin at all. The pin is driven in the space between the end of the pinion and driveline shafts to keep the coupler from sliding up on the driveline splines and disconnecting from the pinion splines.
We'll see how long the repair lasts this time around, but I'll obviously need a new pinion in the future. Since tractors are driven at such low speeds, I'm hoping that it's not a requirement to purchase the pinion gear along with a matching ring gear. We shall see.