If there's rot in the main stem, the roots are rotted. There's no cutting away the bad roots. I would start from the top of the plant and work your way down. The cuttings that root the best are less than 4 or 5 inches long. Let them scab over for a week and then plant the ends in cactus soil with added perlite or, you can lay the stem down on top of the potting soil and snug it down so the stem makes good contact with the soil.
As your other Jades are doing fine, it could be this Jade had the beginning of rot when you got it.
