What is it?
Where does it lead (almost looks like it goes from exaust to exaust?)
Why is it there? What does it do? << not important, just curious

In the 1st pic, it connects to that and runs down under the cab, pulled it out and it was broken








sorry about the huge pics

 

wondering if thats why my truck is running like crap

 

It looks like it is the air pump. It pumps in fresh air into you exhaust to dilute the exhaust. It was the 80's version of emission controls.

 

would that make it run bad? or was it suppose to just hang down there with an open end?

 

shouldnt effect the way it runs unless it originally pumped air in before the o2 sensor.

 

It appears that you have a 91 with a 318...same as mine. The air injection point is after the sensors...down by the muffler.
The air pump itself is a useless, parasitic piece of sh-t. Like crazy4x4rt said, it pumps air into the exhaust stream so that it "dilutes" it so that it appears that your truck isn't polluting the air as much as it really is.
It would be nice if you could take the air pump off and throw it away, but you need it to run the serpentine belt setup. ( I'm working on a reconfiguration for mine...I'll let you all know if it works ) Running the pump probably costs you somewhere around 5 to 10 horsepower.
Letting yours "pump" air under your truck isn't going to hurt anything. The cost to replace the tubing will net you zero in return. Just plug the hole in your exhaust pipe with whatever you have. I used a 1/2 inch pipe plug on my 88 Dakota.
As for your truck running rough, it may be time for a tuneup.