re: "...a conventional "open" (i.e., non-locking) differential, which 95% of 240's have...."

Ha, I would have said 99.99% of 240's.

If you really want one, however (despite the fact that they'll make your car really fishtail and skid in snow and ice), I'm confident that Volvo still has them available -- they've been the same part no. since the 240's began and died, so it's not like they had to stock various versions.

And I've copied the following for you -- I took it from another message, so don't hold me responsible for its accuracy -- I saved it in case I ever needed the information, and am glad to share it with you:
"...the best thing for a 240 or 740 differential is the True-Trac made by Detroit Locker. This is a fairly new limited slip designed for the axles made by Dana Corp. The 240 has the Dana 30, 27-spline axle. This differential as it turns out, is also used in the front of a lot of Jeeps. So it's fairly common and popular, and the True trac is pretty cheap. I've seen it listed for under $400..."