"I am aware that differentials in general are overbuilt, and many howl, yet will function for thousands, or tens of thousands of miles, but I still make long trips in the wagon and do not want to be stranded, or fear being stranded."

Maybe it's of some consolation to you that, since 1999, I'm on my third Volvo and none of them were or are quiet.
1: 1970 Amazon, whined profusely in all gears.
2: 1992 245, slight gearbox whine, esp. in top gear.
3: 1992 245, gearbox whine, esp. in 2nd, and of course top gear.

Unless the whine becomes a really loud howl or increases relatively quickly, the diff would be the last driveline component to worry me on long trips...

In the Amazon I ended up replacing the gearbox and taking the cover of the diff off to have a look. The gearbox was in dire need of new seals, but the gear teeth in the diff showed remarkably few signs of wear. But...nothing I did changed the whine in any way.

Please do compare the sound coming from the diff of your other 240. But my guess: they'll probably be alike and different at the same time, depending on their history. At least that's what I found out after replacing my first 245 with a 100% identical 245 only 800 cars or so up the VIN no. and with similar mileages at the time of purchase.