Good lord. See if your 84 CD has a wiring junction block on the fender well near the battery.
See the pick in post 4 in the attached thread. You Can use a remote starter switch on any of the 2 screws to the rear which have positive and connect to the front one. Or you can less safely use a mumper wire. If that does not crank the engine you have some other issue like perhaps the starter or battery cable issues.
Also, the white wire on the front of the junction block comes up from the starter. With the key off there is no voltage there. With the key on if your neutral safety switch and those transmission bushings are doing their job you will get voltage there.
However, no voltage their can mean your ignition switch is having problems, the neutral safety switch is having problems or your exterior shifter bushings which can sometimes be entirely gone could be the problem or a combination of any or all of them.
Less frequent is I had the inside shifter bushings down inside of the base of the shifter crack and need to be replaced.
Don't be in a hurry to get it running.
http://www.peachparts.com/shopforum/diesel-discussion/261584-jump-83-300sd-starter.html
Also look under the car about where your feet are and see if the chassis to transmission bell housing ground strap is there. Remove it and clean the contact areas. A good idea to do the same with the battery cables.