I have a 99 Expy, with the 5.4l and the 4R100 (AOD), and is 4WD. I believe the tranny is 4 speed? and I am having a weird shutter when it shifts between 3rd and 4th gear. Of course, now as I am typing this, I wonder if it is when it shift into over drive... Anyways, the shutter lasts about 2 or 3 seconds, and sound like you are driving over the center divide reflectors (if they were spaced about a foot apart). Any thoughts? Once it is ion gear it is fine, and everything else shifts smooth and seems fine.