Welcome to the gastroenterology community! First just to correct something you said, the large intestine and the bowel are the same organ. They are never transplanted. Sometimes the large intestine is removed, but there is no need to transplant. You most likely don't have problems with your small bowel/small intestine. These can be transplanted but they are rarely rarely transplanted. I have many friends that will be TPN dependent for life, but they still don't get a small intestine transplant because it is so dangerous.
What tests have you had and what were the results?
Ksoni, I can't address your question on transplantation, but I'd like to make what might be a strange suggestion about your IBS issues. Have you considered checking for food intolerance issues? I work in a medical setting in which we see many with epigastric and abdominal issues of varying severity. We're starting to find one heck of a large majority of the people that come through have food intolerances. There's no one thing everyone reacts to, so some rather extensive testing might need to be done, but it can often be very worth the effort. Consider being check for IgG food issues ('delayed allergy' responses - not IgE issues) and also make it a priority to have total IgA checked - which is your protection in the GI system. If the total IgA is low it could suggest you're actively fighting 'something' and can't mount a further response. But it would also mean you need to have your food -autoimmnue status checked specifically for IgA-gliadin and IgA-casein. Gliadin is to check for celiac disease and casein is to check for reactivity to the major protein in dairy.
Docs who are trained in functional medicine may be the ones to seek out for this type of testing.