Thanks for using PetCoach! I'm sorry to hear about Xena. So if the position you describe (with the hurt leg extended) is when she is SITTING, I am most suspicious of a torn ACL. This is especially suspicious if she will walk on 3 legs but not the injured leg. Staffies between the ages of 1 and 5 are textbook for that particular injury.
If it is when she is standing, that would be an odd way to carry herself and not typical of the usual injuries. (And I would see your vet).
Finally, with this information, I cannot rule out a back injury or disk problem - more likely if she truly cannot stand, even if you help her up and put her back legs under her.
You don't list Xena's age but as I mentioned before, if she is young, some sort of soft tissue injury is more likely. If she is older, an arthritis flare up or other problem may be more likely.
Long story short, I recommend getting her in as soon as possible for evaluation and some safe pain medication. We as veterinarians have so many wonderful pain medications that are safe for long term or occasional use. As such, I would strongly recommend heading to your vet and NOT giving over the counter pain medication (it is not as effective and can interfere with what we would ideally prescribe).
If you are unable/unwilling to go to the vet, probably the “least bad” choice would be aspirin. Be sure to use a product labeled for dogs and use the dosing on the bottle.
See the link below for more information about the safe use of aspirin.
https://www.petcoach.co/canigive/dog/aspirin/Hope this helps and best of luck to you and Xena!