One important lesson I learned is to fill the diff with the pinion pointed DOWN when breaking in new bearings. It will fill up the cavity around the outer bearing. You can also pack the pinion bearings with a compatible grease to ease the break in also.
Little history. I set up my front and rear diffs and wanted to break them in easily. filled with fluids with rear pinion pointed up ~15 degrees. Drove about 5mph around the block and just as I got back to my driveway the rear axle bound up completely. The outer pinion bearing froze to the pinion. Lesson learned. The light oil I put on during assembly is not enough to lube the bearings if you don't drive fast enough to sling oil up in the passage.
When I replaced the bearins I packed them with Mobil 1 synthetic grease. It mixes well with Mobil 1 synthetic gear oil and allows the bearings to be fully lubricated during the break in phase.
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