driveshaft bearing rubber keeps breaking

[jimfulco]

My friend's '75 LWB has trashed another driveshaft carrier bearing. The bearing itself seems fine, but the rubber is broken again. We replaced it about 4 years ago, and again this past October. It's a 2WD half-ton with umpteen million miles on it, with a 350/TH350 & 2-bbl carb. He mostly drives it around town, maybe a 150-mile trip every couple of years. Anybody know what would cause this?

[gchemist]

Check for a broken tranny mount and motor mounts. An out of balance driveshaft could also cause the problem. A driveline vibration may not be felt since the carrier bearing may be taking the beating.