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Welcome, I'm Chris (aka Moose) I'm an independent contractor here at Just Answer.
Most likely the axle bearing is bad causing the seal failure. When you have the axle shaft out have you checked for a wear groove in the axle shaft and checking the bearing rollers for pitting? Also have you checked where the seal rubs the shaft to see if there is a wear groove in the shaft.
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Are you using OEM or aftermarket parts?
Does the seal come out of the axle tube or is it still in place when you go to repair it?
Have you checked the differential spider gears and pinion shaft to see if they are worn out allowing the axle shaft to move inward and outward excessive causing excess wear on the seal?
Yes the bearing does keep the shaft from moving up and down or forward and back, but it does not have any effect on the axle shaft ability to to come out far and go in far allowing the lip seal to roll over when this occurs allowing the seal surface to wear fast.
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