I picked up a dead fox on a call. currently in freezer.
very abnormal behavior. lethargic. shivering, laying in sun all day. Then died. It's either rabies or rat poison.

How do I have it tested??

well if you froze it, the fox can no long be tested for rabies.

Cold not frozen is how you keep until testing. Don't know New York but here, as I assume most everywhere, Public Health Dept is in charge of that. However the mechanics of the process is we usually submit through the county animal control, dependant on the ability of the county, since it's an animal and between the two the sample gets sent off to the University of Georgia for the actual test

Can it be tested for other diseases after being frozen? Rat poison? Distemper?? I found another dead fox on this property.
I have 1 frozen. deep freeze.

Why do you want it tested, curiosity? Unless it fought with a pet or bit someone, who cares? We only have vectors tested if there's exposure; bite, scratch, etc.

Our county health department takes nuisance caught and roadkill rabies vector animals and freezes them until the USDA picks them up for rabies testing. I'm told there are several different test methods but animals frozen animals can still be tested for rabies with certain test methods.

As I understand it: They only need the intact brain to test for Rabies.