2004 F150 4.6 is running erratic, hesitating , and the Speedomter is not working....I'm thinking about replacing the rear speed sensor...does anyone know how I can test to see if this sensor is indeed the problem?

Also while under the truck taking pics of the speed control seensor I found this steel bar hanging on one of the frames under the truck can anyone tell me what the heck this is used for?

 

See if you can Run a HEC test. That will tell you if the Cluster is communicating with the CANBUS. If it passes all the tests it is good and you can move on to the sensors. A little search and you will find out a lot more. The piece of black iron doesn't appear to be painted, maybe stuck up in there.