Ok, from what you had previously said, I was under the impression the box pointed upwards towards the back.
In the case that your box points down towards the back, your pinion angle needs to point up towards the front. Both box and pinion need to be on the same angle. Its the same deal with side to side angle - since the diff pinion is straight forwards, the engine/gearbox has to point straight back. ( Pushing the gearbox output shaft to the side without moving the engine to the side with it, will also result in driveshaft vibration.

Most hardware shops will sell angle finders ( like a vertical compass that measures in degrees )
Just using the eye isnt acurrate enough.