With the clutch disengaged and the truck still rolling, the tranny main shaft is still spinning along with the gears if the trans is still in gear, yes try it in neutral, also both with the clutch in and out.Loose tranny gears or bearings can set up a vibration, generally goes away when you release the gas pedal with the truck coasting in gear ,clutch engaged.If it wasn't allready mentioned, check the bushing in the tailhousing that the driveshaft yoke fits into. Try to move the yoke around to check excessive clearance.
The OD vibration thing is more asociated with the motor mounts being unable to deal with vibration from a low rpm ,high load situation, like pulling a slight grade while in OD.At least that's what I think.
You know Fred, the vibration may have always been there, in the past the low rear end gears probably stopped you from driving fast, now you can and the drive shaft is spinning just as fast as it would befire the OD tanny installation.
