My Ford e350 has started to have the brakes stick while in reverse. Any suggestions?

You need to start with a complete brake inspection but take a look at the rear parking brake first. Things that can cause the brakes to stick are failed parking brake springs, shoes, hardware, calipers, internal issue with master, clink in brake hose. It's best to give it a complete inspection to make sure its safe. Especially since you just bought it. Alex