Q.jade plant is in distress
This jade plant is about 20 yrs old. Lives on my west front porch, gets large amounts of sun. It is looking very withered, although I do water it. Has started dropping larger branches, and the leaves are brown and very withered. When I have taken cuttings, some of the stem has a black vein running thru it. I’m concerned there may be a fungus going on, or maybe it is in the wrong dirt. What type of dirt should I replant this tree in? It used to be about 3 ft tall, maybe 3-4 ft around, but now it is a former shell of itself. I plan to change out the soil this weekend. What should I do?

I suspect that your jade plant is afflicted with root rot. You are making a good call by repotting it this weekend. When you repot it you are going to want to examine the roots. Healthy roots are firm and white. Any roots that are stringy, limp, black or brown will need to be pruned off.
For detailed step-by-step instructions on treating root rot, please visit the following link:
https://www.gardeningknowhow.com/plant-problems/disease/treating-root-rot-gardening-tips-for-housplants.htm
As for soil, cactus and succulent potting soil will do nicely or you can concoct your own mix using equal parts perlite, peat and vermiculite.
For a refresher on the care of jade plants, please visit the following link:
https://www.gardeningknowhow.com/houseplants/jade-plant/jade-plant-care.htm