[Jim B]

Another item to check would be to make sure the driveshaft slip yoke is not bottoming out against the output shaft splines. (Manually move the driveshaft forwards and backwards from the rear u-joint and make sure the driveshaft is not too long or too short) Make sure there are no broken/damaged rear suspension components that would allow the rear to move forward or rotate on the leave. I also assume the shaft length to tubing diameter/wall thickness combination and straightness are not causing a harmonic whip. (mostly seen in an insufficient driveshaft length to tubing diameter relationship) Also any history of cracked flex plates? It wouldn't hurt to check the pinion angle.