Yet another (in a long line of annoying) updates:
Since I couldn't fathom how to reduce the "play" in the driveline itself, I figured my only other option was to try to adjust the mesh between the worm and gear.
Since this is assembled in more or less sequence, it wasn't easy to do. I removed the bracket's front mounting screw, the (2) screws that hold the motor to the mounting brackets, and then loosened the longer rear mounting screw. This gave me enough wiggle room (yet still held everything in place) to add a thin (1/64") washer between the frame and the front "foot" of the bracket, nearest the worm. In the photo the screwdriver is pointing to the area I'm referring to:
Hooked up a couple of leads, and PRESTO! I think that worked! It now runs equally smooth and quietly in both directions: so quiet, in fact, that I have to lean in close to be able to hear any sound at all. Of course, it's not all reassembled and on the test track yet, but I'm hopeful.
There was at one time a discussion on the "Model Train Journal" forum about this model and a similar fix was suggested. I don't know what that outcome was, but my All Nation instructions don't show or mention any "spacers" to be used at this spot...
Anyway, there you go...for now. 
Mark in Oregon
