differential kaput?

[84300DT]

i posted this in the diesel forum. anyhow my indi said the diff on my 84 300D 187k is shot
car makes fast 'clunking' sound at about 25-30mph on and off the throttle. at one point left rear wheel locked up (very scary) but has not done it since . drove her back from the indi and did some test driving. floored car from stop thru all gears - still get the clunking but no lock up , also same in reverse. parking brake works. 1/2 shaft boots all intact/no leaking. diff does not appear to be leaking. i suspect my indi did not take enuf time for a proper disgnosis. any other thoughts out there - i really don't want this car to die but finding a used diff may be hard and many $$ to repair? need expert advice! she's parked right now..