Really does sound like the drive line vibration issue I chased...which I finally fixed with a 3.5" steel driveshaft. My car originally came with a 2.5" shaft and the drive shaft shop said they wouldn't build a 54" driveshaft in anything less than 3.5"...said they'll "noodle" at speed if not big enough diameter.
Hope you find it...I hate vibrations...
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Lance
1985 Monte Carlo SS Street Car
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