i have a 2000 chevrolet silverado single cab, short bed. I has 96,000 miles and just took my truck for an oil change yesterday. i have had nothing done to the truck. Today me and my dad went to see about me getting a y pipe exhaust. on my way home from getting an estimate the truck started to shake violently and hardley run right. Strange noises were coming from the engine. 4.3 L V6. now when i start it it sounds like its going to break down and it shakes in park, neutral, and drive. automatic... please help me someone. tell me whats wrong with it