My dog is obsessivly gnawing at his leg.. he is aggetated and restless... his back leg has been shaking,and when we walk him or he walks across the room he sits down about every 2 feet or so... I have never seen him this way.

Has he been checked over for fleas or mange? Any ticks?

It could be allergies as well. Could be something you used as a cleaning product, or it could be the food he is eating, or it could be environmental as well. Have you done any food changes recently? Any thing stressful happen in the home? Any changes?

Is he exhibiting any other strange symptoms?

I sorry to hear about your pup. This sounds like he's in pain. Have you made an appointment with his Dr. yet?