Oh come on Gentlemen~~~~~!!
It is the SUBFRAME bushings~
Go on Ebay and Type UP: 240sx (subframe collars or subframe bushings)
These bushings hold up your Rear END and after time, these PolyUrethyane inserts wear out.
You can easily replace them for about $40-100 (if you get generic, or COOL KAZAMA!) With these NEW Aluminum "Bushings" or "Collars" You simplY Unbolt the 4 bolts that hold up the rear Subrame and side these "Bushings" in.
You can get OEM polyurethyane that will give you a soft ride, or like most people get "aluminum" or SOLID "bushings"
It takes about 30 minutes to do, put your car on jack stands, Put a JACK under the Rear Differential, SLowly losen the 4 bolts that hold the SUBFRAME (SUBFRAME = THAT ENTIRE ASSEMBLY OR "CAGE" IN WHICH HOLDS UP THE DIFFERNTIAL, THE WHEEL KNUCKLES, THE AXLES, ETC.)
The entire "subframe" will drop, you support it with the car JACK, SLIDE the top collars in, then put the lower collars in, tighten the bolts and you're done.
HERE's the LINK~~~~
http://cgi.ebay.com/ebaymotors...34712