While my '98 2500 4x4 doesn't ride like a Town Car, it does ride quite nicely when unladen, with the Yoko 19. 5" TY303s or the OEM Goodrears.
What did I do to achieve this? I kept the OEM tires at 55 front, 40 rear when unladen, as the owner's manual suggests. I keep the 19. 5s at 65 front, 50 rear when unladen. I ditched the OEM shocks at about 15K miles and installed KYB GasAjust shocks, which, BTW, take much longer than 75K miles to loosen up/wear out. The OEM shocks are, IMNSHO, just plain junk, as they are on *most* vehicles.
Getting the front end *properly* aligned can make the ride a little smoother by minimizing the side-to-side jerkiness and wandering.
Rear suspension changes would help even more, but I'm rather satisfied with the ride as it is now.
Even me 76 year-old Dad likes riding in it for long distances. It's almost as good as his Town Car, but not nearly as quiet! As long as this truck keeps running like this, and doesn't develop any rattles or squeaks, I'll keep driving it. I can't envision going back to a car or a smaller/lighter truck.
Fest3er