Can you share some stories about how Sai Baba's Udi has helped people with illnesses?

Certainly. Chapter 33 of the Shri Sai Satcharita recounts several instances of Udi's healing power. One story involves Khimji Lalji Joshi's daughter, who was so ill she couldn't walk. After being brought to Shirdi, Maharaj applied Udi to her, and within a few days, she began to walk on her own. When she still had trouble getting up, Baba gave her a fruit to eat, which completely resolved her difficulty. Another story tells of a friend who couldn't sleep for eight days but slept soundly after taking the Udi for just one night.