Driveline Hitting underside of floor (at rear)

The best way that I've found to set the correct pinion angle is to remove the drive shaft and put an angle finder on the back surface of the trans tail shaft (where the seal is). This will tell you how many degrees the drive line is pointed down (typically). Then you put the angle finder on the flat (front) surface of the pinion yoke. Sometimes you have to get a piece of flat metal to hold on the pinion yoke face so you have a nice flat surface for the angle finder. This will tell you how many degrees your pinion is pointed down (or up). The pinion should be 3-4 degrees "down" with respect to the trans angle BUT it depends on your type rear suspension of course.
The whole purpose of setting the pinion angle is so that when you accelerate the pinion will rise and get aligned with the drive line (the pinion and the engine/trans drive line should be parallel during acceleration). A pinion will typically rise 3-4 degrees under acceleration "depending on your rear suspension". 3 to 4 degrees is a relative measurement, in the end you set the pinion to an angle that gives you the best results!! Go HERE:
http://www.moparts.org/Tech/Archive/axle/8.html

Remember your pinion WILL rise when you accelerate the trick is to find out "how much" it rises.

Treblig