Transmission Problems

I have a 2000 k2500 that just turned 100k. It has an automatic transmission, that runs fine in 1,2, and 3rd gears. When i put it in D, after about 40 mph I hear very loud racket w/ metal clinging. Like i said i can drive it all day in 3rd and it does fine. Transmisson Fluid is full. I'm assuming this is a transmission problem ?? Some have said driveshafts and differentials, but i figured it would do it all the time and not just in D. Any ideas ???

does this rig have over-drive????
3rd gear ,,, the guts of the thing are all turning as one piece 1to1
ALL other gears require planitary gear sets to achieve output ratios.
if one of the gear sets is in trouble,,,,,may make strange growling
noise,,,which usually goes away when it gets to 3rd.