I don't know about leg pain from my own experience. I have had sciatica in my early 60s but that was well before my IBS D started, and my gut was no problem then.
And even that was fine when I lay down in bed so I could sleep. It wasn't 24/7. But I remember the pain would make me sweat and nearly pass out when I had to do something standing like wash dishes or brush my teeth.
I have a pelvic misalignment though (it's twisted and my left iliac crest sticks out a lot more than my right. The right side is very straight), and some problem with both sacro-iliac joints, and one leg more than an inch longer than the other.
The SI joints hurt like heck every morning, and ease up a lot after a #2.
I don't take any painkillers. Just scrunching into a squat down on the floor, or sitting on my knees "Japanese-style" helps and takes the pain away immediately. So does lying flat in bed or on blankets on the floor. It's getting up again, standing and moving that hurts and my lower back spasms, though I am OK really when moving around during the day and doing physical work.
I don't know how related it is or not to my IBS D.
I hope you find some answers. I would see the doctor and see what they say? Unless you've already done that countless times, and still got no answer...