Found this tonight looking for other info: This may not apply to your case but it could apply to somebody else researching this thread: GROWLING FROM REAR AROUND 70 MPH This is usually due to a worn out rear joint on the driveshaft. Go under and twist the driveshaft joint where it bolts to the differential. There should be no slight clicking or play, but after 30+ years it's gotta go sometime. Unfortunately, the joints on 280Z/ZX driveshafts aren't replaceable. Nissan made them cheaply by stamping the joints in, so there's no way to remove and replace the circips and joints. The junkyard is a good place to get a replacement but you're still stuck with a very used part. The solution is to have a custom driveshaft made, about $300 dollars. Either look in the yellow pages and find a driveshaft fabrication shop, or contact Powertrain Industries. These guys are Nissan specialists and made me a custom driveshaft which was very high quality.