spare tire stuck under trailer

Possibly not Rust. Due to pick-ups losing under-mounted spares, the manufacturers have been forced to place a lock on the cable holding the tire. As the unit goes down the road, the jarring of the highway causes the cable to squeeze down between layers of cable underneath. This creates slack in the cable, which allows the tire to swing. The "cure" has been a clamp that grabs the cable, before enough slack is available to allow swing. The end result is a tire that can't be dropped, without aid from a tire shop or dealership. I'm not sure if trailers have this "feature", but even a little bit of cable squeezed down between the layer underneath can get hung up tight enough to appear to be stuck.