Double check you have seated both the large bearings fully home on the casing. I found when i bought a new diff the casing was very tight where the bearings went on and one wasn't seating full home against the case correct, causing it to get nipped up on the gearbox case when closed up. A quick rub of wet and dry around the GD casing where the bearing sits and it goes on tight, but goes fully home. No more binding.
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