I had a friend who had some back issues, and was extremely well endowed. (I don't know her size, but her girls were ridiculous.)
She tried a lot of things to help her back issues. E first thing that she said gave her significant relief was getting professionally fitted for her bra size. Turned out she was wearing the wrong size, and it messed with her.
She also joined a gym, and a personal trainer (who was free for a month on joining the gym... Lol.) helped her strengthen her core, along with targeting her back and shoulder regions.
Between the right bra and exercise, she got a lot of relief from her back pains, but still occasionally suffered from them.
Ultimately, she did get breast reduction surgery. She is very happy with the results, and her back issues have vanished. (She does still keep up with her exercise routine.) She said the best thing was that it helped her self-confidence, as she was pretty concious about her (as she put it) "mega-boobs."