i have a 66 diesel i'm restoring and there is a vibration in the drive shaft (the shaft between the clutch and the rear end) bothe when idling and when in gear. it's not really violent but it's certainly noticable.

is this somewhat normal or is there something i can try to remady it?  i think the vibration is causing my rear main oil seal to leak too.

any help would be appreciated! thanks!