Are you getting any codes?
If you are not getting any codes then it is likely that you have rust in the speed sensor area and if you can clean it out you will be golden. A coworker had the same issue and he was able to buy new speed sensors for like $15 apiece and clean up the hubs. There is quite a bit of information about this for GM pickups in general since this is a well known issue.
Replacing the hubs apparently WILL fix the issue too but not necessarily needed.