Do you have a diff lock in the front?
I've allways found the Navs will allways perform way better off road with a front LSD/diff lock...as this is where the weight is.
with regards to the rear, d21/d22 and d40s have c200 rears, just with different pinion designs etc etc.
d21 and d22 had options for h233b rears...d22 in the 3l petrol and QD32.. this has a larger housing, larger crown etc etc BUT the LSD shims are similar diameter to the c200 model.
when you compare a patrol h233b you will see the shims are about twice the size and the reason why the patrols LSD are quite famous.
pointing this out because its not overly worth changing to a navara h233b diff for the LSD shims performance etc etc....
I have over-shimmed many diffs, both d21 and d22 and depending how you go it ends up being a lokka. when unloaded a nav is stupid light in the rear, so lokka rear gives you a new driving experience and it will chirp the wheels everywhere...with a bit of weight in the rear it will be easier for the lsd shims to be independent and act more like an LSD instead of lokka.
putting in shims is no hard, if you know a bit about basic mechanics and take your time and do the measurements.
as your diff is playing up, id suggest getting a good condition secondhand one and working on that, as yours wont go back together without proper inspection from a specialist who can set the shims and backlash etc corectly.