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Cool. If the 29 tooth gear fits then it should be a good fix. The mesh is a little lose in CAD so the reason it skips is probably because the axles aren't round, but flat, so when they are on the flat side perpendicular to the gear line they give a little less mesh. Wrapping one layer of aluminum tape around the two drive axles to the wheels will probably tighten it up if the axles are loose in the bearings.

I will upload a test part for the gearbox housing that lowers the top bevel gear that contacts the diff gear by 0,4 mm. I think I made a mistake there which let the gear go up into the housing. The new part is Test01.stl