2002 Pontiac Grand Am ABS, Service vehilce soon, Trac off li

Hi I have a grand am gt and my abs trac off and service vehicle soon lights are on. I have taken it to a dealership and they read the codes and said it was the abs sensor. So I replaced them. I replaced both driver and passenger side hubs twice in one week. The codes kept comign back saying abs sensor. I replaced hubs twice. They dont knw whats wrong with it. Now the lights are still on and they read them again and it says passenger side abs sensor. I replaced it twice. This is getting very expensive with the dealership. Please help. They dont knw whats wrong with it and wont tell me what I can replace to see if its that. Please help thanks.