inside of limb of jade plant is very dark brown and flakey
My jade is about 15 yrs old. I have never had problems with it but now one inside is very dark brown and flaky, part of it broke off. Also, the leaves are dropping as well. It may be lack of water but I am not sure.
If it's 15 years old, you apparently understand the water (or not-watering) requirements very well. As you're saying "one inside," I'm picturing one of the limbs sort of close to the inner part of the plant. Is it possible that it's just the limb developing a more woody growth, which they do as they get older? Then you say part of it - part of the limb? - broke off...do you mean a smaller limb from the flaky-looking part? Could it have been bumped, or had something dropped on it, or something chomped it? The droopy leaves...are these the leaves all over the limb that is flaky, or on the piece that broke off, or the whole plant, or what?
Look the plant over carefully for bugs, especially under the leaves and stems.
Jades don't need repotting very often, but if it's been several years since you've done it, that could be what it needs. If you do repot, increase the drainage of the soil with perlite: https://www.gardeningknowhow.com/houseplants/hpgen/should-you-repot-your-plant-happy-root-bound-house-plants.htm
The only way to be sure of the watering is to test the soil in the bottom of the pot with an electronic moisture mete, or bamboo kebob skewer. When it's time to water, the meter should read about half way between dry and moist, and the skewer should feel just a little damp, with only a couple bits of soil sticking to it.