1985 300td rear end noise

[oldbrown]

This vehicle is in excellent shape with new tires, brakes, and such. Haven't had a bit of problem until doing 75mph on the interstate for about 75-100 miles, then a roar starts to eminate from the rear. Once you pull off the road and slow down you hear a clank, clank, clank, as the wheel rotation slows so does the clanking.
After a short stop and start (say at a stop sign) the sound goes away for another 75 miles or so. I never hear a noise around town, only after a high speed run of an hour or so.

Any ideas?