SCIO, N.Y.—Gunner, a bomb-sniffing dog mustered out of the Marines for canine post-traumatic stress disorder, has found a new home with Deb and Dan Dunham, whose Marine son died in Iraq protecting the men beside him.

With patience and a red-rubber toy, the Dunhams are trying to coax Gunner back to emotional health. With liquid brown eyes and Labrador loyalty, Gunner is giving the Dunhams back a little of what they lost. Together, they are healing what they can and living with what they must.

"My...