When you say “full weight” do you mean all four wheels on their own weight? I was hoping to just use the rear wheels on their weight. How in the heck do guys fit under their car to make the measurement when it’s on the floor?
front wheels up on ramps, or blocks, or Jack stands under the LCA if you have to, and a Jack under the rear axle. That’s for checking the length of the driveshaft. If you’re checking your springs, or ride height, you want it on the tires, and bounced in the front and back a few times and allowed to settle. It’s really best to drive it before you check it, but you can get close by bouncing the front and back and pushing it back and forth a few times.