In laymans terms...
The torqueflite doesnt have a vacuum line like some trannies, but in order to operate, it still needs to know how much throttle youre giving it, in order to decide when to shift, and also regulate the line pressure, or the pressure of the fluid pressing on the bands and stuff. Without it, it thinks youre driving around with the throttle at idle, so it doesnt put very much line pressure into it, and when you give er the beans, you can make things slip, causing friction, cooking the fluid, and worst yet, tearing the material off the bands. Dont drive it without a kickdown, or as its more properly called, a TV (throttle valve) linkage.
If you cant find a TV rod, lokar makes a kit with a heavy duty push/pull cable that will do the same job and allow for a lot more flexibility.
Another thing you can try before taking it out for a rebuild is dropping the pan, removing the throttlebody. Clean the VB out with carb cleaner, clean out the pan (while youre there look for any signs of damage, like chunks of anything in the sludge). While youre in there, get a inch pound torque wrench and adjust both the bands.The procedure can be found in a manual or by google searching it. Then, being very careful to keep everything meth lab clean, even a single hair could get stuck in a valve and make it not work, put it all back together with a new filter and pan gasket, then fill it with fluid, run it in neutral for a bit, then remove one of the lines to the trans cooler, let a gallon flow out, put a fresh gallon in, and rinse repeat 3-4 times. Drive it around a bit to get things spinning, then after that if it still doesnt have reverse or 2/3rd, you can either try putting some lucas transmission additive in it, although this would be a last ditch temporary solution at best, or you know it probably needs a rebuild.