If the "odd whistle" is related only to vehicle speed, then a likely suspect is the driveshaft center support bearing.

I just replaced that bearing in my '86 (with about 270k miles). In the past I've characterized the "whistle" as ghosts singing in three part harmony. My observation is that it tends to be louder when cold, but as the day warms up or the bearing warms, the "whistle" lessens.

The bearing comes in two sizes -- my '86 took a larger bearing, yours might take the smaller size. It's best removed and replaced using a press with the appropriate mandrels and fixtures. Be very careful of the internal splines in the driveshaft. Consider also replacing the rubber "donut."

Incidentally, you can drive a car for years with a failing driveshaft support bearing. I think mine had been singing to me for two years.

Differentials for 240s so rarely fail that they're relatively cheap.
--
Don Foster (near Cape Cod, MA)