I have a 2000 2500 4x4. when the differential is warm and you put it in reverse it makes a clunk. It sounds like a bad U-joint. I had some other problems with the drive line on this truck(see the post for 2500 brakes) but I was told by a dealer that is normal?
Does anyone else have a similar situation?

There is a proceedure for testing output shaft end-play and pinion end-play. The tolerances for these ar a little wide, so a soft clunk is ok. A large and loud bang and clunk could be a worn pinion or crush collar, an improperly setup rear end, or a damaged transmission output shaft bearing or housing.

I'm assuming it's a 2WD truck. If its 4WD, transfer case chain will do this too, along with low fluid levels in the transfer case.

I'm not 100% sure about the ATF+4 queston in your other thread, but I do know if you put in the wrong fluid, you can damage the bands and seals.

I hope this helped you,

Slantsixness

The check ball in the separator plate is usually the cause. As long as the L/R band is at proper adjustment. Should have more miles on it, but things happening.

The ATF+4 is awesome for every torqueflight, Its completly compatable as a top-off or used as routine service. EXCEPT for 1999 and earlier mini-vans with the 604/41TE Transaxle due to TCC statigy, ATF+3 only.

The old Dodge truck I used to have made that noise whenever you would change from forward to reverse. All I did was top up the level in the rear end, and it stopped doing that. That might not be the correction to your problem, but it's a start, and it's something you should have checked right after you started hearing the sound...