What about a roller bed? You know, like they make for a pickup truck. You'd have rollers along the bed, covered by some sort of rubber "track" that a winch or similar motor would just simply rotate the lead roller and you could just drive along as it unrolled them off the back. Or face the rollers cross wise, and roll them off the side.
This roller bed, could be made and attached seperately, and not have to be a permanant part of the trailer, in case you neaded to haul heavy equipment. It would also allow you to haul them lying flat, which I assume you do now.
If not....
A second idea, would be a variation of the cradle ideas. Instead of having the whole cradle tilt, make one side straight, and hinged and when opened, it would serve to protect the tires and fenders, while the other side would be curved and hydraulically lift and roll them over the hinged side and off the trailer.