Your best bet would be to contact guest relations ahead of time or go online to request information. Disneyworld.com lists 407-939-7807 as the number for guests with disabilities to call.
Depending on the severity of the disability, the solution might be as simple as using an alternate entrance to an attraction. There is also a Special Assistance Pass available at guest relations windows -- I believe THAT is for more serious situations, or for "invisible" disabilities (IE autism, that is not necessarily visible to a Cast Member). They usually require a doctor's note and are designed to ease the strain, but do not necessarily allow you to completely bypass the lines. Instead, you might be able to wait in a seating area, rather than a line.
Again, contact the number above and see what they suggest.
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