Seems like I've just been talking to myself for a while, but there has been a little progress made.

I cut down the partial axle that was shown previously, and took a little more care with the diameter this time. I glued the turned stub into the brass tube using a Vee-block to align the parts as the Loctite cured. The original axle shafts were about 101.8mm, I rounded this up to 102mm. I had calculated that I needed to take 28mm out of each axle shaft. But I wanted this "practice" axle to be too long, so I added another 4mm onto my target length. I then cut the 5mm diameter end of the outboard axle stub to length and turned a 3mm OD stub 10mm long on that end. I then slipped the axle shaft together and checked the length with my trusty HF 4" caliper. After deciding it was good enough for "gubmint" work I glued the parts together, again using the Vee-block for alignment
After things had cured for a few hours I couldn't wait anymore and decided to test fit the axle in my 3D printed Frankendiff housing. I had added another 4mm onto my calculated axle length, and there's a 4mm gap between the 12mm hex driver and the outer bearing. So I've got a working procedure for narrowing my WLToys rear axles for use on my Bruder truck RC conversions.
Now I only have to do this at least 4 more times and I'll be able to move on to actually building the truck. I said at least 4, but it will probably be 6 times since I'm 99.44% sure that I'm gonna swipe a very good idea from Blender on another forum and also build a powered steer axle.
Don