This may be old news, but I just thought I'd relay this information to anyone who had the same problem with their truck. I have an 05 Lariat 4x4. Build date was 10/04.

If your truck has had a funny "clunk" when downshifting or coming off the gas on your 2004-2005 F-150, you may want to go to your local Ford dealer for the latest TSB (Technical Service Bulletin). The current TSB will reflash your PCM with updated shift points. It is for all Ford F-150 trucks with a build date prior to 08/2005.

Owners whom have had the upgraded program have reported smoother shifts and better response times. Most importantly, they have reported the "clunk" has disappeared.

http://www.truckblog.com/story-524-p...2005_ford_f150

I just had this done to my truck. I can feel a big difference in responsiveness and acceleration. I am also getting slightly better MPG's.
I have no more lag when trying to pass. Hope this information is helpful.