My 2000 awd ford explorer is clanking, can anyone help?

every time the front wheel makes a revolution it makes a loud clanking sound. I jacked it up and took the wheel off to see if i could find out where it was coming from. There is a rod that i think connects the awd to the back wheels that is making the sound. its more towards the middle than the front and its on the drivers side. It could be the "u joint" or "drive shaft" but im not sure. Can anyone possibley tell me what could be wrong? thanks