Sounds like a leaking rear airspring to me, too. These airsprings have an average service life of approximately 15 years. Some can fail sooner. Any holding air after 15 years are living on borrowed time. I just replaced the two front airsprings on my 89 Mark VII. This was a very easy repair once I saw instructions and pictures.
If the leak is slow enough that you can still raise the rear, at least long enough to jack up the car, then do so. Then turn off the air suspension by the switch in the trunk before raising the car. With the car supported on jack stands, you can spray the rubber portion of the springs with dishsoap and water to check for bubbles to see which one is leaking.
For replacement airsprings, check out
www.bagmasterair.com . They have a good reputation and their rebuilt units are better than OEM and cost half the price. I have heard of other independent suppliers that are cheaper, but their quality appears cheaper, as well.
For additional information on airspring replacement, visit
www.thelincolnmarkviiclub.org or
www.lincolnsonline.com .