I'm struggling with this issue, as well. I've got a 90 4Runner that I'm fitting out for some off-road time in Baja. It has a rear spare, but it's wise to carry two spares down there.
I want to add an undermount spare and a hitch, but all of the hitches I've seen say they won't work with a 31 inch spare.
Can anyone post pics of their hitch and 31 inch spare?
Thanks!