I have decided the easiest way to get "highway gears" in my 37 Packard, without real surgery, is to replace the rear axle. Can anyone point me to a reference for rear axle widths?

The axle dimension reference section on here is bust according to some comments over the years....

Check through here:

http://www.moparts.org/Tech/Archive/axle/17.html

The garbage chart was finally removed.

It is unfortunate that most rear end width charts these days want to show the track width. That isn't very helpful unless you know the wheel width and offset.
You need to know what your width is before you can get one close. Bolt patterns are the next challenge. Finding a solid axle assembly in a pull a part will be the next challenge. Most of the stuff at pull a parts is less then 10 years old.
For what its worth:
S10 are about the most narrow axles.
Early Dakota (pre 91 are 5 bolt, 91 & up are 6 bolt), and Jeep will be the next narrowest.
Ford Explorers are next (and about the best buy)
Then it goes to the late model trucks.
Lots of off shores axle widths in between. Good luck! Gene