There are a few possibilities for what may be going on here. Keep in mind, however, that without me being able to see and examine her, I can only make suggestions.
A "dropped jaw" is an inability to close the mouth. The two most common things that are going to cause this type of condition in dogs is a neurological problem called trigeminal neuritis and the infectious disease, rabies. Trigeminal neuritis has a sudden onset, but is not typically associated with other types of weakness. This problem is of unknown origin, and usually spontaneously resolves within 2-4 weeks.
The fact that your dog has weakness and staggering and gags a bit when swallowing, makes me concerned about rabies. So, unless you are certain that your dog absolutely could not have come into contact with a rabid animal (another dog, cat, raccoon, bat, rat, oppossum, fox, etc...) within the last 6 months....then I would leave rabies on the list of possibilities.
Another possibility here is that the dog does not have a truly "dropped jaw." Meaning, the jaw does have some muscle
tone in it and she can close her mouth somewhat. If this is the case, then the rest of her symptoms may fit with a disease called myasthenia gravis. This disease is usually diagnosed first with administration of Tensilon (edrephonium choride). After the drug is injected, the dog will show a dramatic increase in muscle strength. Definitive diagnosis is made based on a serum AChR antibody titer.
Most dogs are treated as inpatients until adequate dosages of anticholinesterase drugs are achieved. In dogs that have complications due to aspiration pneumonia (if the dog has megaesophagus on presentation), intensive care may be necessary. If the patient is unable to eat or drink without significant regurgitation, they often need a special tube called a gastrostomy tube placed to help them do this.
Although this disease is treatable, most patients require months of special feeding
and medication. The drug of choice is called Mestinon syrup (pyridostigmine syrup). Other drugs that are used are corticosteroids (if there is a poor response to pyridostigmine or if there is no response to the Tensilon test). I do not know of any holistic type remedies that have been found efficacious for treating this disease.
In cases where a mass is found inside the chest (a cranial mediastinal mass), surgical removal is performed. The patient must be stabilized on anticholinesterase drugs before this is done.
Once approprate drug levels of the anticholinesterase have been reached, return of muscle strength should be evident. X-rays of the chest are generally done every 4-6 weeks to evaluate for resolution of the megaesophagus. AChR antibody titers are evaluated every 6-8 weeks to look for a decrease back into the normal range.
If there is no severe aspiration pneumopnia or pharyngeal weakness, then there is a good prognosis for complete recovery. Resolution of clinical signs will usually occur within 4-6 months. If a thymoma is present, there is a guarded prognosis unless complete surgical removal and control of the myasthenia symptoms are achieved.
I am attaching some client information handouts that I use in my practice that discuss some of these conditions in more detail. I hope that you find them useful.Myasthenia GravisRabies
If your vet has been unable to diagnose your dog, then you may want to consider asking them for a referral to see a specialist in veterinary internal medicine or veterinary neurology. Most specialists are located at referral centers or colleges of veterinary medicine. The benefit of seeing a specialist, is that you are more likely to get a quick and accurate diagnosis of your animal's problem, and thus a faster resolution of the symptoms. It is also more likely that a specialist has seen this problem before and treated it successfully.
Here is a link so that you can search for an internal medicine specialist in your area:
I hope that this information is of help to you, and I wish you the best of luck with your dog. Please let me know if I can be of further assistance.