Okay, thanks for the pic. That looks like the rear of the spindle and the spacer's function is to space the two gears a fairly accurate distance apart. It also looks like it has to take some light compressive force as you adjust preload. Finally, the ID is fairly large so you'll have to bore it; something that is easy to machine would be preferable. I don't think heat or shock resistance is a major factor for this part.
Given the above, aluminum or Delrin would work just fine. I don't see that steel or brass would be needed for this part, although they would also work. If it was me, I would just use Delrin and have the part done in a few minutes.