Like TacoSurf said it is really easy to track people today. Make sure you aren’t sharing your location with him on your cell phone. Assuming you are using an IPhone go to your husband’s contact info on your phone and scroll to the bottom. If you see this.
Then you are currently sharing your location with him. Just tap on that and you will fix that problem. Also, change your Apple account password. He could be logging into Find my IPhone from any computer and tracking your phone location that way. If he has downloaded some third party tracking app to your phone you can shut off location services on your iPhone to disable it.
If you shut off your location services for a week and you notice that it seems he isn’t following you anymore then you are going to have to investigate your apps a little more. As far as tracking devices on your truck, check for something plugged into your OBD2, that would the easiest way to place a powered tracking device in your truck. Without a tracking device it should look like this.
But, also like TacoSurf said, there a thousands of cheap tracking devices out there. If you don’t find something in your OBD2, you could be looking under your truck, under your seats, between your truck body and bed for something this small..
They do sell tracking device detectors, I don’t know how well they work but if you find that he isn’t tracking you through your phone or a OBD2 device it might be worth 50 bucks to try one of these.
Before using the detector I would disconnect your trucks battery so you aren’t picking up all the electronics that are wired in your truck. If he placed a non OBD2 device it likely runs on batteries, not your truck’s battery. Good luck and sorry you have to deal with this.