I suppose it would be possible that if the shocks were bad enough that the tires would often lose contact with the road, the system could get erratic readings from the wheel speed sensors setting that code.
If they did the work with the new shocks, they should have cleared the code themselves at the time...not charge extra to do it later. Most codes will go away after a certain number of tests without a failure. I don't know for sure on that one, but I would think it should shut off fairly quickly.
The most common cause for that warning on the Bonnevilles has been the steering position sensor, followed by a bad wheel speed sensor. So far, no reports on bad shocks causing the problem.
Black 2000 SSEi, Silverstar Headlights, Angel Eyes fogs, 3rd brake light overlay, hi-flo cat, drilled 180 degree thermostat, Intense FWI, PCM, shift kit, and 3.4 Pulley. Solid front mount.