What Tani said- check your injectors. Get a stethascope or piece of hose and listento them with the truck running. They should have a noticeable click to them. It could also beyour map sensor. You will need a dvm or scanner to check that. A vacuum leak will lean you out and would only help if you have over feuling issues.If you have been running rich for a while, and have the factory cat it is either fried or close to it.

 

timing could also be off a ways, another thing to check out