I am disabled, and walk on crutches, but do have a wheelchair for whenever necessary. I have visited the South Rim of the canyon three times, but have never been down inside the canyon itself. Are there guided tours down into the canyon for people with disabilities?
Frommers lists that Bright Angel and North Kaibab trails are 'the canyon's smoothest' trails - but as you can see on my picture with a section of the Bright Angel trail, it's not really wheelchair accessible, and probably quite strenuous on crutches. Picture was taken in 2006, so it might look different today.
Another option would be a mule trip, which is possible if you write them beforehand (see "Rider Qualifications").