a833 OD vibration in 4th gear at 3000 rpm

How did you have the driveshaft checked, exactly?

Does it still vibrate when going down the road at the same speed as 3000 in 4th (that must be pretty fast with the OD). Pop it into neutral, let the engine idle, and see if it's still there. If so, it's driveshaft, rear, tires...