What about putting in expandable foam in the drive shafts? Sounds like the cardboard in the new shafts is not the fix! I'm thinking that MAYBE, the expandable foam would be more "solid" and fill the inside of the shafts more completely than just the cardboard unless the cardboard is ground up and packed solid inside? Drill a couple of small diameter holes on opposite ends ofthe shaft so the air can escape if needed and you could also see then when the shafts are full of the foam.(Not sure if the ends of the shafts are open) I would think that possibly the foam would absorb some of the noise coming through the shafts??? Do you know how the cardboard in the new replacement shafts is installed? Is it ground up etc?They have 75/140 in all the gearbox and rear diff for as long as I can remember. So that wasn't a attempt to quit it down. Switch to straight 90w. See if it helps.
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The most noticeable differance from the things that I have tried was the solid carrier bearing mount. I wasn't sure about installing it due to the fact that I feel that the shafts have to have some flexability with the movement of the driveline and the flex of the chassis etc. Not sure that a solid mount is correct.