If you have a rear wheel drive and it's showing code P1875, yes it could be the ECM. You obviously have a 4wd ECM in a 2wd truck. Did you swap the original with one from a junkyard w/o reprogramming it? Or did you remove all the 4wd components from the truck to make it 4wd?
You can just ignore it if you don't mind the SES light being on all the time because your truck doesn't care about 4wd low. If it's an automatic it might shift a strange rpms tho.