DO THE AXLE SWAP. From a safety perspective, and also from a maintenance perspective, this is going to be the better way to go. I'm not a farm boy (I mostly haul boats and boat parts and equipment trailers with my truck), but IIRC hay/straw is not light...and, if you're going to be towing, there's too much of a chance of getting a false sense of security with adapters and thinking you can carry more than you really can, which can have disastrous complications.
You really don't want to be taking chances with this...in the end, it's your equipment and your decision, but in my very strong opinion, you'd be crazy to use adapters on the rear. Get a DRW pickup axle and swap it in.
BTW, I was looking at doing a DRW swap on a factory SRW pickup, and was considering adapters...someone who knows a lot more about these trucks than I do spent quite a while convincing me that I was better off getting my hands on a DRW pickup axle and swapping it in. After learning the differences between the two axles, I'm very thankful that I listened to him.
If you don't know the full extent of the differences between the two axles, I would strongly suggest doing some research on the subject...
