Hello and welcome to Just Answer. I am a licensed veterinarian and would be happy to answer your questions.
Is he a dog that has had itchy skin problems in the past (less severe)?
Have you ever used antihistamines with him?
I have seen dogs so uncomfortable from skin problems that they exhibit all of the signs described by you. You are right however that it is impossible to differentiate it from a pain response. Whatever the cause, it doesn't sound like anything you will be able to offer over the counter will be enough. If you have emergency services available, I would suggest that.
Good luck and please let me know if you have any further questions.
I agree, it is difficult to tell. The shaking is a concern and may be pain related, like from a disc problem (like a pinched nerve).
There are not over the counter pain meds that are safe to give, but it would be ok to give a dose of non-prescription Benadryl to see if it calms him down. The dose is 1 mg per pound that your dog weighs so a 20 lb dog would get 20 mg. This can be repeated two to three times per day. The pill that is available over the counter is typically 25 mg. so you may need to go with the children's liquid form which is typically 12.5 mg/teaspoon (or 2.5 mg/cc). Make sure that there are no other "cold-type" medications in it like decongestants.
If he is itchy or uncomfortable due to skin problems, it may help. It also makes them a little sleepy so that may be enough to calm him down. If the shaking continues, I would have him seen this weekend.
Constant licking of anything around him can indicate a seizure like condition called psychomotor seizures. It can also be seen as a type of obsessive-compulsive behavior.
I work with a veterinary dermatologist and we see a lot of intensely itchy dogs. It is not uncommon to see them get almost "fit-like" in their behavior when they are having an itch flare up.