The tension control rod is the part of the front suspension which keeps the lower control arm from moving forward and backward. Actually, it causes it to move by a specified amount around a fixed point at the front of the car. At this point there is a bushing, the only alternative being a ball joint, as it must flex in two directions at once.
The stock tension control rods bushings are made of rubber and filled with silicone gel. They are vulcanized into a metal ring which is pressed into the T/C rod. They can be replaced either by replacing the entire T/C rod assembly (as the dealer will not sell you a stock bushing alone), by installing Nismo T/C rod bushings, which are firmer than stock but made the same way, or by installing an aftermarket bushing which is typically made of polyurethane. There are also T/C rods which use a spherical bearing, which is the hardest fit possible. However, this will transmit more vibration to the suspension, and thus to the brake system, wheel hub, and so on, so it is recommended only for race use.
It costs about the same amount to buy T/C rods with polyurethane bushings from PDM Racing as it does to buy factory T/C rods. However, if you have access to a press and can bring a little bit of ingenuity to bear on the problem, you can get just the bushings from them for $50 (by far the lowest price I've yet seen) and press the old bush out yourself, clean the inside of the T/C rod with some 500 grit sandpaper (I sanded it wet and by hand) and then press in the bushings with the included grease.
I have heard that polyurethane bushings tend to squeak, which might be true, but certainly is not happening to me with these. It might not occur until the grease has been through repeated temperature cycles, but after installation they are certainly quieter than any other noise you might be able to hear.
Below follows a brief description of the T/C rod replacement process, with some pictures showing what I'm discussing at the moment. You can click on the pictures to see a (MUCH) larger version.
As you might suspect, you should start by loosening the lug nuts on the front wheels, and jacking up the front end of the car. You can probably do this one side at a time but in my experience front suspension work is best done with both wheels off the ground to minimize annoyance from the sway bar. I have two jacks so this was extremely easy, Kragen tends to have specials on them all the time and I picked up two jacks, four stands, a folding lug wrench (neat!) and a creeper for $65 in all. This plus one more lug wrench is enough for two people to get a 240 up in the air and stable in roughly no time at all. You can do it fine with the factory jack, but you're going to need jack stands and if you get the stuff on sale, you can get a decent hydraulic jack and stands for about what you'd pay for stands alone.
Once you have the car up in the air, you can move along...
This is the original factory T/C rod. Note (in the large version) that it has cracked and the rubber has separated. This is actually the opposite side of the car from the rest of the pictures, but I removed the passenger side rod before photographing it. I flipped the picture around so they'd all look like they matched. Start by removing the nut from the other end of the bolt whose head you can see in the picture. It's got a 17mm head on it.
This is a picture of the top of the other end of the T/C rod, where it bolts onto the lower control arm. Note the guard on the T/C rod which is doubtless intended to save you from immediately swerving off the road in the unlikely event that the bolts holding this end should shear off. Next to it is the sway bar end link. Remove the two 17mm nuts from the bottom side (identical to the nut for the bushing end) and you can now remove the bolt from the bushing end, lift this end up, then pull the rod toward the back of the car to remove it from the bracket in the front.
This is a picture of the old T/C rod next to a new one. Note the spot where the metal has been ground down - this is what happens when you get too carried away while driving a lowered 240SX. No serious damage was done, but it would definitely be best to discard these T/C rods rather than reusing them. Note that the nuts are on the T/C rods; not doing this means you will probably damage the threads. Any time you aren't doing something which requires the nuts to be removed from the studs in the back end of the T/C rods, keep the nuts on.
Installation is simply the reverse of the above process. You need to put the bolt back through the bushing end before tightening these nuts down because it will make it much harder to keep everything lined up; When the suspension is hanging down, the bushing is twisted somewhat. When you tighten the two bolts at the back end of the T/C rod, the rod will rotate and load the bushing.
Now just torque everything down to the proper specs which can be found in the FSM - I didn't do this or I would have the figures available. I just tightened the hell out of it, and then checked it later from the values in the FSM.