Did you upgrade diff bushes at the same time ?
That could be increased NVH that you are hearing.
I had a Quaife in my 3.7:1 diff that was quiet, but since installing 3.9:1 gears repalcing the OEM buses it is noticeably noisy but has covered 12k klics and not getting any worse, so I just turn the stereo up a bit.
The KAZZ in my 4.08:1 diff with the same bushes is less noisy.
So, it could be increased NVH from the bushes if they have been replaced, or it could be diff shims on pinion or crown-wheel which is up to the shop that installed the diff to correct if it is bad enough.
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