Okay, whoever had "side rods" wins their choice of cigar or milky cocoanut...
As you can see, it runs silky smooth on just the rear axle.
The question at this point: Will the loco pull, say, 5 cars with just one driven axle? If so, I can remove the side rods entirely and let the front drivers coast; the client is more interested in it running smoothly than 'prototypical' appearance. If not, it's time to requarter the drivers...
Mitch