Just to verify, had you recently changed the diff balls? This could also be symptomatic of metric 3mm diff balls instead of 1/8” (3.125mm)….the smaller balls would result in the rings riding directly on the face of the spur gear. A thinner spur wouldn’t be the issue, but a thicker one definitely would be. Then, correspondingly the stack up as a result of the smaller balls could make you run out of thread when trying to tighten the diff… just a thought.