The Authorization for Use of Military Force Against Iraq Resolution of 2002 was a Constitutional declaration of war.
A resolution authorizing military action is not a formal decalration of war - they are two distinct things.
The title of H.J.Res. 114 was "Authorization for Use of Military Force Against Iraq Resolution of 2002". No where in the legislation is a formal declaration of war stated. War was not declared against Afghanistan, Vietnam, or Korea either. The US has formally declared war against 11 countries: UK (War of 1812); Mexico (Mexican-American War); Spain (Spanish-American War), Germany & Austria-Hungary (WWI); and Japan, Germany, Italy, Bulgaria, Hungary & Romania (WWII)