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vibration after driveshaft swap

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1991 AWD AUto XT6

the closest ujoint to the transmission was bad, so i replaced both halves of the driveshaft with one with new ujoints. the joints on the replacement driveshaft were all good and moved fine when i installed it and i even greased them since it has new greaseable ujoints on it. this driveshaft ran fine in my last daily driver with no vibrations at all. now it vibrates in my new XT6. the balance weights are still on it.

 

if it is a balance issue....the front one can't really change how it's mounted, the carrier bearing is what it is. so the rear half is the one i can change. what should i do, i have three options:

swap it around, end over end

unbolt the front (by carrier bearing) and rotate it 180 degrees

unbolt the rear (diff side) and rotate it 180 degrees

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Clean up all the mounting flange faces really good..a little bit of rust between parts can really throw balance off badly. Do all the u-joints seem to pivot freely and with about the same amount of resistance?

It really shouldn't matter which end is which, or how the two shafts are "clocked" in relation to one another..they should both be balanced assemblies, mixing & matching shouldn't cause unbalance issues.

 

You could always try the old hose clamp trick if all else fails..once you figure out which shaft is causing you the grief, put a large screw-type hose clamp (like a radiator hose clamp) on it..tighten the clamp up, take the car for a drive. If the vibes are still there, rotate the clamp 1/4 turn and tighten it up again..keep doing this in smaller increments until the vibration stops or there's a noticable decrease. Like balancing a wheel, only you use the "screw" part of the clamp as a movable weight.

If you can't make it completely disappear with one clamp, add a second clamp..

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that's what i thought, i've swapped numerous shafts before with no issue so i was surprised to see this happen. the joints are fine, they are brand new and freshly greased and worked fine in my other XT6...so i can't imagine them being the cause. not much rust at all to speak of.

 

i'll try changing the orientation just because it's easy and check on rust/seating of the flanges while i have it off.

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cleaned the driveshaft up and all the joints are perfect (they are brand new and freshly greased). swapped it end for end and still the same thing. you can't change the orientation on these, the holes are such that they only bolt in one way...so i guess you could rotate it 180 degrees, but you can't change the orientation of the ujoints.

 

drove it with the rear half of the driveshaft removed (in FWD) and no vibration at all, so i'm assuming it's that half that is bad. there are no places around me that balance driveshafts so i'll have to find somewhere in maryland or ship it somewhere to have them check it out.

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