you can adjust your angle via the transmission cross member,
( not too radical though),,if you can attache a plumb line or string to align with your oil pan,,and bring it back along the oil pan,,then to your transmission,,to rear end,,,you will get a good indication of your angles.
Your driveshaft may be out of ballance, causing the vibration.
Simple things like universal joint wear will also give you vibration,,,especialy if your angles are wrong.
Need more info on your rear axle setup to be further help.