Some years ago I has a similiar problem with my 4.9. Check and make sure the throttle blades are returning all the way back to their proper resting position. The hardened steel shaft on the throttle body had worn into its soft aluminum bushings causing it to hang open just a little bit. The T.B. can easily be disassembled and if the wear isn't too bad, you can smooth the bushing surface with some fine emory cloth. There are a couple of o rings on the shaft which can also leak air and raise the idle. They can be replaced if they are hardened up.