Would it be a poor choice to focus on disability in both my personal history statement and diversity fellowship response?
The two sections are read by two different (if sometimes overlapping) committees. The personal history will be read by the general admissions committee. They might note issues of diversity and disability which can lead to a fellowship, but if there is a separate question for that -- they may assume that that information will be duplicated there. Still, if your disability played a significant role in shaping who you are as an academic -- the kinds of questions and approaches, or the empathy you bring to particular subjects or students -- I would include it in your personal history statement as that will speak to your motivation for study. 

The diversity / disability fellowship question is also important. There are many small fellowships in the UC system and faculty try very hard to ensure that students get every penny they can -- but this is only possible if students self-declare. I would not feel shy about repeating any information in the diversity fellowship response -- again, you want to ensure that you have the funds to study what you want to study.