can someone explain how not reassembling the driveshafts as they were, could cause a vibration? my car came with new flex discs installed and a driveshaft in the trunk. i didnt notice it when i bought it, or else i would have asked, but im wondering if someone replaced something under there and thats why i have a spare shaft now. i ask this because i get a vibration at all speeds. start to feel it around 30+. it feels more engine based than rotational/drivetrain, since it will actually increase with throttle, even before the car accelerates from that increase in throttle. i'd like to rule out things before i change engine mounts and engine shock.

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1980 300D