Old struts can harden up sometimes, depending on what component is failing. How hard is it? Are we talking typical small car firmness or "OMG, this thing feels like a go kart" harshness? What were you driving previously? 89k miles really shouldn't wear out a set of struts, unless there's a lot of gravel roads in the car's past. I'm actually thinking you should go over the suspension, looking for worn bushings, bad wheel bearings and the like. A bad bearing can cause noise and vibration that can increase apparent ride harshness. To check, lift up a wheel and grab the tire at 12 and 6 o'clock. Try to pull it in and out. You'll feel some movement, but you shouldn't feel any slop. If you do, the bearing is shot.
Hope this helps.