2000 navigator: sit higher on one side than the other.

This sounds like the beginning of a long long list of problems to come. If it were me I'd just go ahead and convert the car to coil suspension. It's cheaper in the short and long term and you won't even have to think about it for the rest of that car's life.

1997 2002 Lincoln Navigator 2WD Rear Air Suspension Conversion Kit With...

The air ride suspension has level sensors under the vehicle, one of the sensors must be bad making the truck deflate the airbag or inflate it too much. you may need to have someone with a scan tool to access the air suspension module and check for fault codes. you can try turning the air suspension switch off for about an hour to see if the system will reset. the switch is located under the dash on the right side kick panel.