I especially like the last line from this article,http://datsunzgarage.com/probs/
CLUNKING REAR END This is the bane of all Zcars, it's almost impossible to completely fix. It can have many causes but it's rarely caused by the internals of the differential, especially if it's an R-200. The problem is that if you tighten things in one spot it seems to immediately shows up in another: these cars will find any slack in a heartbeat. If there is one thing that Datsun should have designed differently it's the rear end on these cars.
If you hear it thumping the floor behind your seat it's a broken/stretched differential mount...the rear driveshaft flange can actually jump up far enough to strike the handbrake linkage. This sounds like a ratcheting sound on hard acceleration. If you hear it farther back its likely one of the causes below:
- u-joints on the halfshaft ends are loose and worn-out (common, replace the joints)
- tighten the bolts/nuts on both ends of the halfshafts
- driveshaft bolts loose
- worn out mustache bar bushings (replace with urethane)
- shocks bad or loose in strut
- replace the control arm spindle pins
- ball bearings/grooves in halfshaft body worn (replace halfshaft)
- tighten the two large nuts that hold the differential to the mustache bar Even after you try all these you may still have a clunk. The only solution then is learn to shift more smoothly.