Hi, I am Dr. Peter and would like to help.
1- Can you describe what is happening with your dog's back end?
2- How long has it been going on?
1, when he lays down, his back end just starts to shake like he is cold or something.
2. this has been the second day.. he is eating and drinking.
1- Is he lame?
2- Is walking different?
3- Is he having trouble getting up from the floor?
2. only with his tail down
3. no, but having touble climbing up stairs
1- He is walking with his tail down?
2- Can he wag his tail?
he can wag his tail yes
The shaking is probably secondary to discomfort (pain). The signs of sometimes carrying the tail down and difficulty going up stairs implies the pain may be coming from the lower back and less likely the hips. I would recommend that you have him checked by your vet and placed on some antiinflammatory like Rimadyl. If you do not have access to a vet then you can give short term of Buffered Aspirin 5 mg per pound every 12 hours with food. Do not use for more then 1 week as it can cause GI irritation.
Consider also supplementing with glucosamine & omega 3 fatty acids; these are very safe, you can pick up over the counter. They help with inflammation and decrease pain associated with musculoskeletal disorder. They are very safe and take about 4 weeks to see benefits.