Good job going through a chinese truck and getting it shimmed to perfection and buttery smooth. It sounds to me more like the gears need time for their mesh to wear in. Probably run a couple packs through it and recheck.
Also might want to scuff up the friction surfaces on your slipper clutch while its apart.
Or being chinesieum, it could have an off spec bearing trying to act more like a One-Way beaing.