88 240 rear end vibration

Have you checked the lube level in the differential yet? The symptoms sound like it may be low.

Mike

Does the vibration happen specifically at the speed you mentioned, or does it also happen whenever the engine is at a certain rpm? (regardless of what gear the transmission is in) If so, and given the age of your car, I would suspect the bushings in the rear end are bad, namely the torque rod bushings. These bushings exert force on the car whenever you are pressing the gas or pressing the brake. Does the vibration go away when you get above a certain speed or rpm? If so, that's a pretty good indicator that it's bushing related. The vibration of the engine is being transferred to the body of the car and the car is resonating with it at certain frequencies. If the vibration does not go away above certain speeds/rpms but gets louder/changed pitch then I would look more at the rear differential.

Thanks for your responses. The torque rod bushings were actually changed last fall but none of the other rear end suspension bushings were changed. I'm not sure what rpm's the engine is at when this noise is present (no tach), but it seems to the worst at around 80 km/h. The noise quiets down when I get up to 110-120 km/h, but it could also be masked by the wind noise (which is much louder at that speed!). I will be on the highway later this week and will double check this. Plus, I will definitely also check the diff oil levels to make sure those are good (I'm keeping my fingers crossed that everything is ok in the differential!).

Sounds like the symptoms of bad trailing arm bushings to me. These are the bushings directly below the rear axle.