Al, This is not a total lock up, but near for side to side play. The four axles being allowed the ‘excess’ side play is to allow the locomotive to negotiate an 8 foot circle (4 foot radius). It is also a prime reason (according to TOC) for the tendency to derail in the early part of a curve. The loco enters the curve and the lead axle side slides and as the second axle enters the curve the lead axle ‘snaps’ back toward center creating the unstable condition.
As for the spacer, keep in mind that there is likely to be lubricating oil from the mail bearings and from the rod pins that will find it’s way into that area. If using any kind of plastic or resin, be certain that it will tolerate the lubricants you use on those rotating joints.
Hope this helps some.