My 2000 4.6 truck gets a clunk sound when I get off the gas at around 40 MPH. It doesnt happen at any other speed except 40. I went underneath to check the U-joints and they seem fine. I also notice that when the truck is cold it doesnt do it, but in about 10 minutes the clunk will be there. Does anyone have the same problem?

 

Is you're truck a high mileage rig? What you are describing can be differential side bearings that are worn; allowing the carrier to shift from right to left in the axle housing. This would be more prominent on deacceleration (getting out of the throttle). Noise would be less noticeable when the vehicle is cold (diff. lube would be stiffer).

To check for diff. bearing side play: remove rear cover and insert a pry bar between back side of ring and axle housingand pry, taking note if play is observed--then try the non ring gear side for play.