One thing I have found and it was already mentioned is that more commonly,rear springs go soft. So empty the trunk first! Roll the car back at least 6 ft and then forwards again. Park the car on a flat concrete area.
Roll a floorjack under the rear diff. On the saddle should be something to let the rearend rock from side to side. Something like a steel ball would be nice but I have used round bars, any size. So center the bar under the diff and get the wheels off the ground.
Now go find a reasonably level point in the wheelhouse lip, and measure down to the ground. Repeat on the other side from the same reference point.Compare the two. Rarely will they be identical, but I suppose a quarter inch tolerance, to as much as 3/8" would be OK. This is the time to crank the T-bars up and down. Crank the high side down and the low side up, in equal amounts. Then jounce the suspension, measure, and repeat until you are satisfied.
When the front is thus leveled, drop the rear end down.Roll her back 6 ft, and forward again. Then jack the rearend back up on the balance pin. Now off to the front and remeasure. Adjust as necessary. Repeat as often as necessary to be satisfied.
Drop the rearend down. Jounce her a couple of times.
Now eyeball her. Remeasure the fronts. If they are not level, when they just were, it is because the rear is crooked.Go verify that.