a sagging T-bird
Assuming that there isn't just something flat out broken, the sag is most likely simply worn out springs. When you put air-shocks on, I can understand why the ride wouldn't be the best. Those shocks would be doing their normal shock absorber thing PLUS taking up some of the structural support load that springs and all should be carrying.
My recommendation would be to find a spring supplier somewhere near you. Sometimes new springs would be available...sometimes they are available 'rebuilt.'
Springs from a wrecking yard may or may not fix the problem...after all, they'll be old too and may sag just as much.