Like my 75 J-10, the cable run's back under the body to the low range unit that is attached to your Quadratrac transfer case. When you pull the cable it engages the low range. When pushed in it disengages it.
As far as replaciong the cable, just go get any push-pull cable of the correct length,and diameter, swap it out and you're good to go.
If you're really tired of the cable, find a 1976 or newer FSJ with the Q-Trac and low range, and adapt the lever style shifter to your truck. The lever will go through the floor right next to the tranny hump by your right foot below the front of the seat.
Are you saying you have a shifter on the floor that has 4 LO on it also? If that's the case, someone has removed the Q-Trac and installed a different case. The cable shift to Low range is only found with the Q-trac, and is a seperate unit that bolts on to the left hand side of the Q-Trac case. The unit is about 10"-12" in diameter and about 9"-10" in length.
If someone has swapped T-cases on you,(to other than another Q-Trac), the Low-Range cable is useless.