All you said previously was that you "opened the differential", not "I disassembled the whole front axle including the brakes and found the friction discs are in good shape". I know that the differential can be removed from this particular axle without complete disassembly, so what was I to assume from your statement..?
Please give good information if you want assistance to save us all time.
In that case, change the oil for 80W/90 EP to SAE50 TO-4 and use the anti-squeal additive I mentioned above. See where it gets you.
A question. Has the jerk got worse over time or did it just appear suddenly..? Are you sure you found and replaced all the damaged parts in the front differential..?