Drivetrain clunks on start-out

'98 Ranger XLT supercab 4X4, 4.0 with 4spd auto tranny, 99K miles. Almost since new there's been a clunking noise from the tranny or driveline when starting out from a stop. Have had it to shops a number of times and replaced any suspiciously worn rear end parts and axles and serviced the slip-yoke. There is also an occasional pinging noise that I thought sounded like a worn U-joint but all seem tight and functioning. The clunking is now getting more noticeable and can be felt through the chasis. So, back to the shop today -- still no answers. Checked all motor and trany mounts; they're fine. Trans tech said the posi-trac plates (clutch?) Are worn and can be turned by hand, but didn't think that would cause the noise. Ford tech said it would be the dry slip-yoke but regreasing with the special grease makes no difference. I'm afraid it's going to let me down on the highway and would like to come up with the solution; any suggestions? THANKS!