that part has nothing to du whit hard shifting. it direkts power between the front and back wheels (from 100% back to 50-50% front and back)
and it is mounted to the transfear case and not the gearbox.
if you have shifting problems at high rpm i say check your clutch.
a clutch that donīt fully dissengage make for hard shifting.
perhapse as simple as change your break/clutchfluid (same container)
and blead your clutch at the slave cylinder.