Hello there!
Has your dog been seen by your veterinarian about this problem?
Unfortunately, there is no easy remedy until a definitive diagnosis is determined. Shaking and muscle spasms can be due to a long list of issues such as, but not limited to the following:
1. Pain from a variety of causes including: Tick borne or other infectious diseases, musculoskeletal pain (spine, joints, soft-tissue injuries, etc), stomach or abdominal pain, urinary tract issue, and more.
2. Metabolic problems such as: Addison's disease, liver problems, and more.
3. A seizure disorder.
4. Congenital or inherited problem.
5. Behavioral Problems, and many more.
It would be best to have your dog examined, have complete blood-work (including lab work for tick and heartworm diseases), urinalysis, and X-Rays, etc. if necessary,
If all the tests are normal than an idiopathic seizure disorder, or behavioral issue can be addressed since both diseases are determined by exclusion.
Once the cause is determined there are herbal and other holistic treatments that can be used either with conventional therapies or used alone that will help, but trying to prescribe any medicine without a diagnosis is a shot in the dark.
Bach Flower Remedy's and essential oils such as lavender can be used to help calm your dog while your are waiting for the exam, lab results, etc. 50mg of Benadryl at bedtime can be continued until a diagnosis is determined.
Also....lamb and rice foods are easier for them to digest....a scoop of dry with a tablespoon or two of wet makes them very happy! There are also lamb/rice bones for treats. Treats are in the morning, meals in the evening.
My sister's maltese as well as my bichon has problems with twitching....sometimes to the point of not able to walk, dragging one of the hind legs. My dog just started this last month, my sister's dog began this about six yeras ago. We give them bone and joint powder 1 scoop mixed with a bit of water and pour over their food. It seems as long as we continue this regimen they are fine. Also, by keeping a sweater on them when they must be outside in the cold helps. Not sure if this is the same problem yours is having but you may want to try it....and by all means, mention it to the VET to rule out any health issues that should be treated. (we also use 1/4 tsp liquid benedryl to their food which seems to help, however once it's under control we do not continue). Best of luck to you and your Lab.