Hi Phage,
I am told that tylosin/Tylan is now on the FDA Veterinary Feed Directive List, which matches what you found: it is no longer OTC, but is available under the direction of a veterinarian.
Do you know of any poultry veterinarians in San Diego County? You can pm me if you want.
@artblueggwanter
I checked in with the UC Davis Veterinary School Poultry Extension specialist, and he pointed out that the clinical signs could reflect bacterial or viral infections which require a different course of action, therefore seeing a poultry vet is necessary to appropriately diagnose and treat the underlying problem. This benefits your flock directly, and others in the future.
Are you familiar with the CAHFS labs?