Clanking noise in drivetrain (1970 4X4 c10 4spd).

[Boxxman]

I'm new to this site. Came here completely baffled by my 1970 4X4 C10. There is a significant noise coming from what sounds like right beneath the cab. It happens when torque is let off, or applied to the drive train (shifting gears/letting off the gas). I've had buddies watch the driveshaft where it enters the transfer case and the CLANK noise is perfectly in sync with the application/removal of torque. That said, I've had three shops look at it. The first changed the gears in the rear differential/U joints, rebuilt the clutch (4 speed BTW). The second shop swore they found the noise in the front lockouts in the hubs and replaced those. I just picked the truck up this morning from a transmission shop (the first two weren't just tranny shops); who said all was tight in the tranny AND the transfer case (205PT) didn't know what else it could be. I've got about $2500 invested in all of the above and the noise hasn't abated at all. It drives great and shifts great BUT Sounds like crap in the on/off torque situations. At this point I'm out of options, each of the three places that worked on it are reputable, but still couldn't fix it. Of all the c10's out there, I'm hoping someone with a fix will recognize my description. Please help!