Full body weakness and mild paiin after tetanus diphtheria shot. Even after 72 hours. Is it serious?

Probably not. Flu like symptoms may happen for few days after vaccination, it usually clears by itself, some tylenol (acetaminophen) and good hydration might help. If symptoms prolongs after a week, have it checked out.
Vaccination reaction. It is very common to have some reaction to vaccines, and these include the symptoms you describe. Taking Tylenol (acetaminophen) and drinking plenty of fluids may be helpful. If your symptoms are not getting worse after 72 hours, then you should not be too concerned; however, if the weakness gets progressively worse then you should see a doctor to make sure that you are not having a rare serious reaction.