I have a 2004 F150 5.4 with tow package. The trans seems to be working normally with no noticable slippage, but their is two thuds that feel like slop in the driveshaft or rearend.
This occurs on slow accellerations only, is this normal
Thanks,
Steve in FW TX

 

i would crawl under it and inspect your u-joints, just grab your driveline and twist it whilst in park with parking brake set. see if it moves, its gonna move a little but your looking for movement in excess of normal play in the tranny and rear end. see if any parts or locking rings are missing. but sounds like a bad u-joint to me.

 

Could also be the splines on the driveshaft, where the driveshaft attaches to the trans and if it has a center support where they connect. Try lubing those locations which may require removing the driveshaft, if it doesn't have zerk fittings.

 

my parents truck is doing the similar thing mostly when turning slow.they where told to change the rearend fluid and put some kind of additive in. dont know if this is the same deal or not. so just throwin that out there

 

I assume this has a limited slip and what you are feeling is clutch chatter. If it is under warrenty they will replace the clutches if not try changing the rearend oil and add the friction modifier.