Yeah, known bug - Welchol can interfere with all the fat soluble vitamins. I take Welchol and vitamin D - I take 5,000 IU's a day. My levels were very low, now they are ok. I try to avoid taking them at the same time, but it doesn't seem to matter. They check my D levels every 3 months.
I can't do the soluble fiber. Ugh.
I would also try a tanning bed, maybe talk to a nutritionist. There might be foods that would facilitate absorption.
I've probably said this before, but a few years ago, taking extra vitamin D was considered dangerous, and we were all advised not to exceed the recommended 400 IU daily dose. Now they say it's probably not really a vitamin, maybe more like a hormone, and everyone needs lots of it.