As far as I know even locked axes have some elasticity resulting from solver accuracy. You could try to increase position/velocity solver iterations for those meshes in their physics properties. Also, you could try setting projection to some very low value at risk of getting jitter. Also, adding another set of joints will also help stabilizing as that is an equivalent to more solver iteration for joint. Damping in joints or actors could also be very useful to avoid that constant vibration (maybe in the second set of joints where limits could be soft?). Have you found any solution?