Steve, from what you described, I would DEFINITELY take the driveshaft assembly to have balanced. The chances are excellent that the PO pulled the driveshaft apart without marking it and put it back wrong, so it's no longer in balance. That could be a big part of your vibration problem right there.

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'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.