I just bought a new alum driveshaft and had the same problems you have described. After a loooong time troubleshooting this vibration, and trying various things, I decided to get one. So I will let you know if this clears it up!
Good luck!
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Joe!
1988 GT, 167,000 miles!!! 13.58@105mph Check out my listing!
Click here! Or my website:
www.joe4speed.com
1999 Ninja ZX-6R 10.32@135mph!
1993 Olds Eighty Eight LSS 16.40@88.8mph