Allen, the vet called back. Even he has feed allergies as 3rd on his list for the cause of hives in our area. The first are gnats, the second is a contact allergy, like change in bedding.
In the case of gnats, he said to look for "crusties" under her belly and other delicate areas -- negative. There has been no change of bedding, and the hives are restricted to her neck, which could have been in contact with something that bothered her, but they are not on her legs. They are on her sides and withers, which are covered by her blanket, so if it is a contact reaction, complete contact doesn't seem to be necessary. But there have been no new changes in her environment. Bedding is the same, nothing new in her paddock. I'm flumoxed.
So, if not a food allergy, then it's a mystery. The prescription is benedryl and I'll feed a flake of the old stuff tonight, just in case, and start again tomorrow.