While disassembling the Big Rock front/rear diffs, I found a washer/shim that is not depicted on the parts list or the exploded view parts diagram. If you think of the diff assembly as a ring and pinion, this shim is on the back side (no teeth side) of the ring, before the bearing. This shim is only in this location and only on the front and rear diff. I contacted support, they were puzzled when they found the same shim and reached out to Arrma. Apparently this shim was added last min as a running change to tighten the gear mesh of the diff. FYI.