Pinion angle question.

LEAVE IT ALONE and run it. LOL

pinion 2* down gives you a -4.5 pinion angle. A little higher than ideal for a streeter, but doable.

If it's vibrating on decel, that's because the pinion will dive further negative stressing the u-joints, so maybe roll it up 1 to 1.5* and see if the vibration reduces or goes away.