Re: I have what appears to be a big time issue.....
I have to agree that you don't want that trailer on the road again and that you need a new trailer, however I disagree with floating the boat off that trailer, I think the best idea would be either pay for a small crane(depending on the weight of your boat) to lift your boat off that trailer and put it on your new trailer or crib the boat off the old trailer and slide the new trailer under the boat. Time wise a crane would be quick and very easy so your talking the crane operators minimums which can be fairly pricey unless you know someone, cribbing the boat off the trailer can take a bit of time and materials if you don't have them so its personal choice but I think once you consider time and materials the crane would be worth the money unless you have a yacht...
The reason why I wouldn't do what Joe suggested is that the frame is broken but has not completely separated, I would be worried that during transit the frame would completely fail and possibly damage the boat hull. If you can weld the frame (big if seeing that the frame is obviously weakened by rust so you may not be able to weld it anyway) up a bit to support it then I would think that it has one final trip to the Marina in it, problem is that now that one side of the trailer has failed you have no idea how much stress is now on the other side and I think its safe to assume the other side of the trailer is in nearly the same condition as the side that failed making it pretty sketchy.
Your trailer has served its lifetime, kind of sad to see her back broken like that..
After rereading your post I thought you would want a little more info, what happened is rust intrusion of the base metals of the trailer, over time rust has dissolved the metal frame of the trailer until it broke at the trailers weakest point, where the bolts for the axle leaf separator mounts to the frame. If I were to guess either you or a previous owner spent some time in salt water and thus you see the long standing effects of salt on metal that is not properly protected. It is repairable but not cost effective, its metal you can replace all the beams in that trailer if you want but your going to have more into repairing it then it would cost for a new trailer which is what the others are try to say, it is not repairable with the boat on the trailer were talking a complete rebuild of the trailer and therefore your just better off replacing the trailer and getting scrap money for the old gal.
The frame main beams started as one piece unfortunately that is no longer the case for your trailer.
Do you boat on Salt water? this is important because a galvanized trailer would be much better suited for you if thats the case
