My 2004 F150 has a clunking sound in the rear end when I accellerate, what could be causing this?

 

There is a 99% chance it is the center splined slip joint on your driveshaft.

Put stands under the rear end , crawl under, draw a stripe through the slip joint (so you put it back the same way you pulled it apart) take off 4 bolts on rear yoke , pull shaft down and back , coat splines with the heaviest sticky grease you can find , put it all back together, and it will be silent again for about a year,...it is possible you could have worn ujoints though.

 

ojai150 is probably right, but there really are many things that could cause this including objects loose in the truck bed, a loose spare tire, loose shock mounts, broken or loose springs/shackles, etc. If we knew the exact circumstances of the clunks occurring, it would help a lot.

 

When my 2004 150 with 95,000 on it started making that clunk on start off , I took it to a transmission shop , and they tried everything to make it go away , even though they had the trans out a couple months before , and I asked them if they lubed the splines heavy .
they rebuffed that suggestion like I was an idiot , and ended up taking the diff cover off and trying to tell me the ring & pinion were worn out (yea right)
They buttoned t back up and filled with new grease and charged me $90 for their "expert" diagnosis ,... Apparently they deduced that maybe I knew what I was talking about , because it was silent when I left , because they had simply coated the splines with heavy grease.
35,000 miles later , it started clunking again , I pulled shaft and coated it with the heaviest old Mckay grease I had now at 154,000 still quiet.