Yes they do make a difference on a "non-lifted" truck. I have a dual cartridge kit on my 98 Expedition(4X4,Towing Package);which was installed after I got some 20" wheels.
I first installed such a kit on a 1986 Ford Bronco, and I liked the difference in handling. I had also installed polyurethane bushings (where appropriate) around my undercarriage.
I got my cartridges from J.H Whitney (check under Suspension/Handling), but there wasn't a specific mounting kit for the Expedition Series, but I used the same unit specified for the F-150 Truck. I made my own brackets since none were available for the Expy's.
I do very little "off-roading" ;since I am primarily a high-way driver. However with the poor road conditions in the North East;particularly NJ, and PA the need is still there for pot holes, and ruts and bumps. They don't really cost that much for either a single or dual cartridge kit, and can help soften the blows to the pivots, and joints.
