Well, I think I have indeed confirmed that some kind of soybean intolerance is possibly the source of my IBS-C. I just went soy-free for over a week and the symptoms cleared up completely. I was even able to go off Miralax and fiber supplements. But today, I slipped up and had a side dish that had some soy sauce in it, and tonight the gas/constipation has returned, albeit in a milder form than before.
Is it common for IBS-C to basically go away when certain food triggers are avoided? My doctor seemed not to take the food intolerance issue all that seriously, and he implied that I was probably stuck with IBS-C for life. I'm glad I looked beyond his advice and pursued the food intolerance experimentation on my own.