The rear Falkens showed up today.
I am also getting a good handle on the vibration that I have been having above 75 mph. I have narrowed it all down to insufficient engagement between the output shaft and the slip yoke. The lack of engagement is allowing enough up and down movement of the yoke to cause a vibration. I got a new Strange Engineering slip yoke and tomorrow I am having a new driveshaft made that will slide the slip yoke further inside the transmission. There are some pictures and more discussion on the subject here:
http://www.chevelles.com/forums/showthread.php?t=232582
Andrew