I left the factory spare from my 91 somewhere between Massachusetts and Pennsylvania on i84. Really wish I knew where, or maybe I don't. It took the last inch or two of the cable and the crossbar thing with it when it left.
Other MJ still has the factory spare up there, horribly rusted, dry rotted, and probably flat.
I think they just crank up against the frame, never had any issues aside from when that cable broke and it left without me even realizing it. I did manage to fit an almost bald 32" BFG MT up there, the stock cover on my swapped 8.25 scuffed the tread when the rear leafs compressed and I decided I should start carrying it in the bed instead. A 31 would fit no problems.