Oh, man, Adrian... that IS a lame question!
No... just kidding... actually it's a very GOOD question. And you're exactly right. UHaul
had them commissioned from three or four different factories for their rental fleet. They had them done in the 13' size (the most common) and the 16' size and they were in production for three model years, from '84 through '86. A few are titled as '87s.
They rented them for a few years, and then after deciding they weren't worth keeping, sold them off to the public. I don't believe that there are any solid production numbers available, but I remember reading somewhere that someone estimated that there may be as many as 2500 of the 13' trailers produced and probably only a couple hundred of the 16' trailers.
trailers use the same frame and chassis components as the UHaul
cargo trailers which made it easy for UHaul dealers to carry and stock spare parts as necessary. Unfortunately, all of the chassis components on a UHaul are proprietary which limits their exposure to theft, but also makes it difficult to find wheels etc. today. Fortunately, axle
and running gear is replaceable with industry-standard parts when it's necessary.
I believe that the only replacement parts that aren't readily available are the taillight lenses.
On edit... I see that Donna beat me to the punch!