I have been experiencing a hard thud that can be heard and felt inside my 01 F150. It happens after reaching speeds of about 15 or more and then coming to a stop. Anybody know what it is???

 

You're describing the very problem I had with my 1993 2wd Ranger.

Some F150s, (AFAIK), come with a two piece drive shaft system. My aunts 1998 4wd 5.4L did. Anyway, what happens is the slip joint isn't very well engineered, there's too much difference between each end of the slip joint. If you do have a two piece drive shaft, take it out, (marking each mating end) disconnect the slipjoint and grease the ever living mother out of it. That will help.

Basically what happens, is the slip joint tends to lock up above speeds at 15mph, and doesn't allow slippage as it should. When you slow down back below 15, it suddenly slips again, and it does so HARD. That's your problem.

Like I said, grease the ever living mother out of it. If it still does that, I'd be looking elsewhere. Hope this helps.