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Sadly your mechanic is correct as the driveshaft is hollow and meant to be sealed with a metal disc plug at the gearbox end
this disc can break away and then allow ATF to run down the inside of the shaft which washes out the heavy grease used to protect the joint
the 1st image shows the new shaft with the plug in place
and teh 2nd shows a shaft joint with the displaced plug
so the only solution is to repalce the shaft with new as its very likely the joints are also damaged as teh ATF doens't provide enough protection
do you still need help?
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