After reaching out to MIP...
From the MIP install instructions https://www.miponline.com/assets/images/mip-inst-18140.pdf
Previously I had installed neither of these spacers because this is not a Traxxas Slash install and the rear hub seems to have a machined face that the rear wheel bearing sits against. These are XO-1 rear hubs, which are for all intents and purposes "hopped up" Slash hubs, so I did test install the 2mm spacer (blue arrow I added) this time since I had it on hand anyway. It fits, and it spaces out the *inner* races of the rear wheel bearings. I put the 1mm spacer (red arrow), and removed the 0.5mm spacer I had previously between the outer wheel bearing and the hex nut. With the hex nut cranked down as hard as I can now, the whole assembly rotates completely free like the front, and there is zero play in and out. MIP engineers 1, me 0. I still did not install the 2mm spacer they have shown at the inner most end of the axle to the transmission. This is for spacing on a Slash and would need to be much, much thicker to serve the same purpose on our longer differential output drives.
Rear wheels now spin free of any binding whatsoever (all the way back to the spur gear). Very nice, tight product compared to other axles I've used. Big thank you to MIP for these and their tools that we use (and their tech help time). This set of axles I have is an older set, as the picture depicts it uses the obsolete inner hub design. They clarified that it's just an issue of a change in manufacturing they made and I don't need to go replacing the inner hubs unless I manage to break one, there is no performance or reliability difference between the new design and the old design.
From the MIP install instructions https://www.miponline.com/assets/images/mip-inst-18140.pdf
Previously I had installed neither of these spacers because this is not a Traxxas Slash install and the rear hub seems to have a machined face that the rear wheel bearing sits against. These are XO-1 rear hubs, which are for all intents and purposes "hopped up" Slash hubs, so I did test install the 2mm spacer (blue arrow I added) this time since I had it on hand anyway. It fits, and it spaces out the *inner* races of the rear wheel bearings. I put the 1mm spacer (red arrow), and removed the 0.5mm spacer I had previously between the outer wheel bearing and the hex nut. With the hex nut cranked down as hard as I can now, the whole assembly rotates completely free like the front, and there is zero play in and out. MIP engineers 1, me 0. I still did not install the 2mm spacer they have shown at the inner most end of the axle to the transmission. This is for spacing on a Slash and would need to be much, much thicker to serve the same purpose on our longer differential output drives.
Rear wheels now spin free of any binding whatsoever (all the way back to the spur gear). Very nice, tight product compared to other axles I've used. Big thank you to MIP for these and their tools that we use (and their tech help time). This set of axles I have is an older set, as the picture depicts it uses the obsolete inner hub design. They clarified that it's just an issue of a change in manufacturing they made and I don't need to go replacing the inner hubs unless I manage to break one, there is no performance or reliability difference between the new design and the old design.