死体：An objective way to say dead body; used in law or police investigation for example, when they simply mean “dead body” without referring to any specific individual or showing respect to the specific individual.

遺体／亡骸：More of a subjective and respectful way to say dead body. It refers to a body/shell after the soul left so it’s more personalized than 死体. If you are talking to the survived family of the dead, for example, you would be using this. 遺体 is probably used more often than 亡骸, which is slightly more poetic maybe, but the meaning is the same.

遺骨: This refers to ashes after cremation.

死骸: Usually refers to a dead animal

The definition of 死体 and 遺体 are almost same and both of them mean the body of dead human, but there’s slight differences.

死体 is used to express just a object as a dead human. On the other hand, 遺体 is more polite way to express dead human, it has respect for the dead human as a person.

亡骸 also means the body of dead human, but it is generally used when the body without soul of sprit of the person should be stressed.

遺骨 means the bones of dead human after cremated.

死骸 is generally means dead creature, and rarely used to express dead human.

I do not have 100% confidence for my interpretation, but I hope it would be plain explanation and helpful for you.