Yes it is an option. The brain for the keyless entry/antitheft alarm is up under the dash kinda behind and to the right of the dash gauge cluster. You can probably see it from looking up under the dash or you can for sure take the gauge cluster loose and push it to the side and see the control module for the system. I believe you can give the Ford dealer your VIN and they can tell you every option that your truck came with.
Whenever you have the truck shut off and the driver's side door open does the red "Theft" light, just to the left of the steering wheel, blink on and off? If so you've got the factory alarm and keyless entry. When ever you arm the system the "Theft" light stays lit untill you disarm it. I've never had a problem with my keyless entry but my alarm doesn't always work.