Installing these lift a brand 3" lift cups in the rear of my grand marquis.
I had already purchased and installed them but there was more squeaking in the rear than I was comfortable with. I also took out the axle and had it rebuilt using a P71 Limited Slip Differential, 373 gears, and, police axles. I like that it's lifted and retains the factory smooth ride, and with the air lift 1000 bags, I can adjust stiffness, a little bit of ride height, and pump some more air in them if I decide to tow something. As a safety concern, I decided to weld the cups to the axle, so they didn't fall out while air borne. I also added a 1" tall piece of 2"od 1.75"id pipe to keep the springs in place on top of the spacer. I highly recommend running the rubber grommet/bushing.
Hope this helps someone.

Disclaimer. I am not a certified technician, just a guy that builds cars for fun. Work at your own risk!
Be safe, have fun, keep the greasey side down!