Chevrolet Repair: 04 1500 Silverado running rough, o2 sensors, lean condition

Question
I have an 04 1500 Silverado 5.3 with almost 29000 miles. While running at idle the whole truck shakes, not violently but a lot more so than my 2001 5.3. If i rev the engine up to 1500 rpms or so and let off the rpms will drop down in the high 300 -400 rpm range lights will dim, blower fan loses speed, and the truck quivers but then comes back out of it. It runs rough while driving as well and is especially noticeable while using cruise control. I guess you could say it seems like it stumbles. All this occurred after I had plugs changed to Bosch +4. It ran rough and mileage dropped so I put factory plugs back in but it has gotten progressively worse. I was only able to get about 9.5 mpg driving highway. I took it to the dealership but they claim it runs like a top. It has not thrown any codes and I don't know what to do! please help

Answer
if it gets worse when you use cruise control, and you have cables for the throttle, that means you have a vaccuum system for your cruie. i would check your vaccuum lines for a broken one, or leaking one. you may have bumped a line and cracked it, or disconnected one to make it easier to get to a plug and forgot to put it back. from what you tell me, you use a vaccuum controlled system, and it gets worse, to me that means you have a leak, and the system is running too lean, which would explain the rough running, the attempted stalling, and the bad mileage. the o2 sensors are seeing a lean condition, making the truck run bad, and tells the computer to bump up the fuel delivery, making your mileage worse. so, give that a shot, also, have your o2 sensors checked out if you cant find a leak. that could also make those conditions happen. let me know how you make out.