I've taken in crutches and places them in the overhead bin on CRJs, although that wasn't with UA but in Europe. They fit, just make sure you preboard so that there is space, and expect to get off last.

EDIT: This was using forearm crutches as that's what we usually use in Europe, in the US I've seen a lot of what appears to be called "axillary crutches", which probably won't fit.