In my experience, the cause of rock-hard lymph nodes (not all, but a few) turned out due to low-sodium diet (yeah, I was stupid enough to fall for the propaganda, without realizing that it was addressed to people who habitually eat lotsa processed foods, which had nothing to do with me).
Anyway, after having those rock-hard nodes for over a decade, I got them nice and soft --unexpectedly!-- by reintroducing salt in my diet. And the way I did it was doing a saline purge before a fast. This entailed drinking a gallon of slightly salted water, about 6g per liter -- this is not a typo: not 9, but 6 g per liter. I used good quality sea salt. I did it weekly and after only 2-3 weeks was astonished to discover that all those rock-hard nodes were gone. They have never returned.