What is causing that noise
 

2000 f150 v6 auto 100k miles. When I put truck in drive and reverse and excelerate hard you hear a loud clang that immediately disappears. Any opinions as to what is the cause? Just got new rear brake drums and shoes installed by a dealer. Dealer noted noise from u-joint but I didn't notice the noise prior to the repair

Its a shot in the dark here but it kinda sounds like slack in the driveline. Possibly from worn U-joints.

As for not noticing the noise I didn't notice my lower ball joints squeaking until my mechanic showed me the state of them during an oil change, now I hear them literally every time I get in my truck.

Can't imagine "a dealer"- presumably a FORD "Dealer" rebuilding your rear brakes w/o road testing. That kind of Hands On Experience is difficult to 2nd guess, especially based on your brief post.

The original UJ's are Lubed for Life & many folks never "notice the noise" they commonly make as they're wearing out. That is, until they finally blow out a set of shredded needle bearings. If you "put truck in drive and reverse and excelerate hard" often enough you can rapidly accelerate UJ wear.

Have you heeded what the "Dealer noted" & bothered to inspect your UJ's? Or do you think the "Dealer" is incompetent & FTE contributors are clairvoyant?

As Club Wagon suggested, best to just inspect the u-joints. It's not that hard to remove the drive shaft, if a defect isn't blatantly visible, loose cup/pounded out needle bearings and check for binding of each joint.

Drive shafts are balanced , Put a mark on the shaft and rear end before removing the bolts!

Does the clang sound like a shovel hitting a rock?