Outer Limits 4x4 Board

When I am in 2wd mode I could hear a clunking sound on my rear diff on 1st and 2nd gear and not on a high speed. Also doesn't happen if engaged on 4wd high and low.
Any suggestion on what could be wrong.
It is just a matter of adjusting the bearings by means of the right and left adjusting nuts. Is a shim necessary to fix the pinion depth as well.

mmmm first I would have said maybe bushes for the springs or allowing the diff to move around, or maybe shock bushes. Happens under high torque, so would happen for so in 4wd......

You'll to give us some more info. Vehicle? what type of noise? ( I know - a 'clunk'), noise on or in diff??? Does the tailshaft/pinion move by hand, in a 'lateral' sense?

I asked the diff mechanic to add a shim and change the oil and no more clunking sound.