Checking the diff without taking the cover off is difficult as any of the gears in there can cause a clunk. Get it up on axle stands get the wheels well off the ground and see if you can replicate the noise by going from forward to reverse and braking. Get someone to drive or watch and listen for the noise or possibly see some movement on another component. Diffs rarely seem to go wrong but if you can’t find the noise it’s worth trying. You have checked the oil level. It may even point you in some other direction. Be careful,be safe if you haven’t got the equipment to get it off the ground don’t bodge or get underneath.
As for the callipers as you drive forward and brake it tends to keep the callipers at the top end of the bolts if you understand what I’m saying and if you reverse it tends to move it down because you braked. On selecting forward gear the movement is less severe so less noise. Best way I can explain.


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