Is there a device which will allow tracking on a gps unit?

You need a system to receive the GPS location of your trucks. Check with a 2-way radio dealer in your area. I used to install these systems when I worked for a radio shop. We set up several companies with GPS tracking for their vehicles.
Don’t expect real-time tracking like the special effects you see in the movies. It can be done but would be very expensive. Depending on your needs, you normally receive a vehicles location every 10 to 30 minutes. You can also poll a vehicle any time you want to see where it is if the system is between polls for that particular vehicle.