Have you checked the reaction rod bushes? They have a short life in my experience, only about 40,000 miles, but are fairly easy to replace. At least you can unbolt them and replace the bushes on the bench. I keep a spare pair with new bushes ready to go, and then its just a few minutes work to swap them over. My tip is only to remove one rod at a time, to keep the axle roughly located.
They are two horizontal rods running forwards from the top of the axle casing, bolted to the chassis side members, about 2' long.
If the bushes have gone soft then the axle can swing round under torque, causing the nose piece of the diff to hit the bump stop.
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