"A valid common law marriage
exists in AL when there is capacity to enter into a marriage, present agreement or consent to be husband and wife, public recognition of the existence of the marriage, and consummation." Waller v. Waller, 567 So.2d 869 (Ala.Civ.App. 1990). See also, Hudson v. Hudson, 404 So.2d 82 (Ala.Civ.App. 1981).
There appears to be nothing in your stated facts demonstrating that a common law marriage exists between the girlfriend and your late father-in-law.
You would have to complete the probate, which may not be worth the value of the truck. However, if you already have title in your name, then you can have a repo company hotwire the truck and remove it from the property.
If you do not have title to the vehicle, then you may want to consider offering the girlfriend to agree to sell the truck and split the proceeds 50/50, because something is better than nothing.
It really boils down to the value of the truck, because perfecting title in your name may be not worth the cost, and if the girlfriend can't perfect title, either, then she's stuck with a vehicle that she cannot title in her name. Better to be stuck on her property than on yours.
Hope this helps.