The hitch was not actually traveling at 75 mph relative to your car. You and the hitch were both going the same speed when it fell off. It slowed down due to friction with the ground and the air, but its relative speed with respect to your car was probably 20-40 mph (a very rough guess).
Sorry I haven't responded to the ad quicker, I have someone local who was #1 in responding who wants it. I've sent PMs to the other people who inquired, and will get back to you if the local guy falls through.