The driveshaft in the '63/'64 was pretty crappy. It was a two piece construction with a "damper" - a rubber biscuit - molded between the pieces. Knowing how these were made makes it highly suspect.
Additionally - normally a driveline does not have enough mass to upset the vehicle increasingly as vehicle speed increases. It relys on a "resonance", resulting in a characteristic like you describe: it increases to a point and then gets better with increasing speed.
Tires, on the other hand, have enough mass that typically once they start they just keep getting worse the faster you go.
These two facts would lead me to start by looking that driveshaft over very carefully.
K
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'63 LeMans Convertible
'63 Grand Prix
'65 GTO - original, unrestored, Dad was original owner, 5000 original mile Royal Pontiac factory racer
'74 Chevelle - original owner, 9.56 @ 139 mph best http://www.superchevy.com/features/s...hevy-chevelle/
My Pontiac Story: http://forums.maxperformanceinc.com/...d.php?t=560524
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