My rear brakes are, occasionally, making a bit of a groaning noise and grabbing harder when stopping at low speeds, especially (but not always) when wet or cold.

The only rear brake maintenance I have in my files are a new pair of cylinders installed two years ago. I'd think it'* time for a brake job, but nothing sounds like squealing wear indicators or grinding drums, and I have plenty of brake fluid in the master cylinder.

Do you think I need to re-adjust the rear brakes, or should I start shoe and drum shopping?

Drum brakes last like forever. Mine have 110k on them. I pulled the drums and still have half the pads left. I would say pull one off and inspect it. If the drum looks good, and the pads still have enough left, then put the drum back on and check the other side. But if the drums are grooved up, and the pads are nearly gone, then change them all out. Best to get new drums and shoes, turning the drums isn't worth the cost when you look at what new drums cost.

drum brakes grab harder when it'* cold. My dad'* blazer would brake hard on start up and first couple of times the brakes were used. But not a bad to check them. to see if your brakes are dragging just raise the wheel and give it a spin

Fuel mileage isn't down, so it might just be wet shoes. Plenty of water in the roads today.

Whatever it is, it came up suddenly, so in terms of severity, I'll put it between "wait and see" and "annoyance."

I'd pull the drums and check the adjustment is correct, sometimes the self adjusters dont work, check for weeping cylinders while they are off, a small leak wont show up as a dramatic fluid loss at first,
Wet shoes, slack adjustment and fluid on the shoes will give similar symptoms, check them out anyway - your brakes are your main line of defense in avoiding trouble.

Coming up suddenly when we get a ton of rain, it'* likely the weather. Wouldn't hurt...tooo much to pull them and inspect. I cracked up at your quote.