unbalanced drive shaft

["Robert W. Murphy" <rmurphy@domain.elided>]

Can someone please describe what an unbalanced driveshaft feels like when 
driving?  I have this vibration I feel in my lower back while driving my 90 
Spider.  I can feel it during acceleration and at certain constant speeds.  I 
can temporary eliminate the vibration by slightly adjusting the throttle pedal 
while I am driving.  I don't think this is caused by motor mounts or u-joints. 
 All have been replaced in the last 2 years, so I am thinking that the 
driveshaft is unbalanced.  My mechanic says that the Spider driveshafts 
usually don't go bad or need balancing?  Any thoughts?

Rob Murphy
90 Spider
Coral Springs, FL

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