Australian National Helicopters at Alice Springs Airport, in NT Australia have a pretty good set up if my memory serves me correctly.
I saw them towing a B47 Soloy with a F250. The trailer itself supported the aircraft on some sort of airbag self leveling suspension. If you don't know what the roads are like in the NT, well they are pretty crap, apart from the main highway most of the roads are gravel or dirt, and aren't graded that often. Really, if you are looking for a set up for an Ag work, it might pay to give them a call. I know they didn't have any problems with the aircraft from it being carted around on the trailer.
Sorry I couldn't be of anymore help.
Try looking up Australian National Helicopters on google australia.
Cheers,
Shifty