the driveshaft was designed "out of phase" to eliminate vibration. Probably has something to do with the off set of the engine. This '69 ad actually mentions it. .
When I had mine rebuilt there was an older man (couple years older than me) who remembered there was "something weird" with Firebird driveshafts and said it should be rebuilt the same way.
![[Linked Image from i601.photobucket.com]](http://i601.photobucket.com/albums/tt95/920-680-1902/firebird/Photo_2006_1_29_22_27_15_edited_edi.jpg)
