Nice discussion we've got going on here, deeply appreciated.
Blender:
I use Simplify3D for my slicer and I can scale all the axis independently if I want. I've tweaked the basic setup of the printer, it's a Wanhao D6 clone by Monoprice, so that I get accurate external dimensions. The diff halves should measure 120mm across, the circular part of the housing should have a 37mm OD and it should be 7mm thick, they measure 120.18mm, 36.98mm, and 7.21mm respectively. The first 2 are close enough for me. I'd like to get the Z height a little closer, but since my version of S3D does not allow for variable layer thickness, I've got to accept that the thickness could be off by a layer thickness, 0.2mm is what I'm currently printing at. External is fine, it's the internal pockets that are the problem. Drilling/reaming the 9mm outer bearing race pockets wouldn't be a problem. The only way I could see to get to the inner 12mm pockets would be some sort of line boring set-up, and I've only got the 6mm hole for the axle to work through.
Zabco:
I've been using the default set-up for the STL's, which I think is 4 degrees. Since I've been poking and prodding at all 4 of the parts, and have to reprint them all, I'll look at making a higher resolution STL.
Frizzen:
This is a replacement WPL diff for the "1:16" scale trucks.
https://www.ebay.com/itm/22618175378...Bk9SR-DG5t3EZA
I don't remember the exact part number, but this is REALLY similar to what I am using for a donor axle.
https://www.ebay.com/itm/20485224934...3ABFBMkKPg3cRk