You need a multi-pronged attack: heal the bowels to improve digestion/absorption, control or slow diarrhea so nutrients have a chance to be absorbed, eat highly nutrition/easily digested foods and weight train to gain healthy weight (muscle, not fat).
Supplements you might try for intestinal healing are George's Always Active Aloe, Mucosaheal, l-glutamine and omega-3s to reduce inflammation. L-glutamine is also excellent for controling/slowing diarrhea.
There are several posts here with dietary recommendations and several good books out there: Breaking the Vicious Cycle, The New Eating Right for a Bad Gut, Listen to Your Gut. Nut butters and avacado are good sources of healthy fats. Check out Absorb Plus shakes as a healthier alternative to Ensure, which is full of sugar and unhealthy fats. Absorb Plus was developed specifically for IBDers and is predigested, so even an impaired system can get the nutrients it needs.
If you're not already weight training, start a gentle, total body program and work your way up to a heavier/longer workout. This will help combat the bone loss of pred, as well helping you gain healthy weight.
Diagnosed with ulcerative colitis spring 1999
Therapeutic dose sulfasalazine.
Probiotics, l-glutamine and fish oil caps. George's aloe vera juice and Mucosaheal.