Pinion angle....with pics
If you need to raise the car to get under it, either come up with some ramps/ blocks, even 2x4's to put under the tires, or use jackstands under the rear axle and suspension. IT IS important to have the car somewhat level because you want the car weight distribution in some sort of order as it would be on the ground. As Cracked says, measure the angle of the engine/ trans in such a way that it is accurate to the crank/ trans shaft centerline, and then measure your pinion angle and compare the two. Neither has to be level to do that, you only need to compensate for how far off level the two are, IE the difference between the two.
If you end up with actual 2* I would guess that is a starting point.