I had to replace one of my ABS sensors after having CV Joint/Swivel Problems. But I suppose it is possible that the Sensor could have come loose or dislodged. The Sensors (atleast on the front) are cylindrical about 1/2 inch accross, by about 2 1/2 inch deep, and disapear into the Swivel Housing. check that one isn't obviously seated higher than the other, if it is a slight Tap with a rubber hammer may reseat it. probably best to get it put on a Diagnostic computer first to indicate where the problem lies.
A good Independant will do it for a reasonable price, mine did it for free.
Main dealer will charge you an Hours labour for plugging the Computer in.
Good Luck.
Jubby