l have a 99 Navigator and the driver side rear looks like it is leaning. l was told by a local mechanic that the air spring needed to be replaced. However, when doing research online l saw an overwhelming list of air suspension problems involving height sensors, compressors, relays. . .etc. Can anyone shed some light on a couple ways to better diagnose this problem?

Like l said in your other post, rip the junk out and swap in coil springs.

Most people prefer to save ~$2000 by doing this rather than swap new parts in just to have them crap out later on.

Trust me, I've seen Crown Vics with this issue. Just swap in coils. You will not notice the difference, but a fatter wallet.

jack the navi up and releave some pressure from the air bag. get a spray bottle with water and spray on bag. if it bubbles there is a leak in bag and can be replaced. it takes about two hours to replace one and they will last for quite a long time. the bags get warn like anything else on vehicle and sometimes just needs replaced and it will be like brand new. http://strutmasters.com. try this site. it will get you a price range for the bag. not to bad on wallet.

a few things to check one is soon as u start the SUV listen for the air pump it should just run a short period of time maybe 30 to 60 seconds & stop & not come back on. If it runs a long time u have a leak. If it runs normal then u might have a height sensor bad. Also after starting & if it airs up & sit level then turn off the air suspension & drive it that way & at the end of the day & the next morning see if it goes low. Now this whole thing about swapping out the air suspension for coils if u do not tow any thing or do not carry heavy loads then swapping out is a good idea u can buy a kit or find in the salvage yard a Ford Expedition that did not come with the air ride & use the coils off of that. But if u carry a heavy loads or tow u should attempt to keep the air suspension functioning.