2003 silverado 4x4 vibration

I own a 2003 chevy silverado and in the past month I have a vibration which occurs at 900-1100 rpm. At first I thought it was in the exhaust but now I am begining to think that the vibration might be traveling. Has anyone run into this? Any help would be great.

Does it occur sitting still in park or neutral??

No, the vibration is noticed when driving under load starting from a complete stop or with you foot on the brake and keeping your RPM between 900-1200. I have been told to check for the exhaust gaskets. If they colapse that the flanges from both the manifolds and exhaust will rub metal on metal which will cause a vibration.

My 03 2wd does the same thing. At about 40-45, the rpm is at the right point to cause the vibration. Driving me nuts! :banghead:

Just a thought...fan/fan clutch?