Driveline vibration!!! I need help!

If the pinion seal is leaking I would look at the rear. The pinion could be twisted or bent. This would cause a leak and a vibration. Take the driveshaft off and have someone spin a wheel and look a the center of the pinion, or put a dial indicator on it and check to see if it is bent. I would say it going to be from the trans back. Steve