A federal court in a diversity case applies federal procedural law, but must apply substantive (includes choice-of-law + statute of limitations) rules of the state in which it sits.
If there is a federal law unrelated to the cause of action that is not a federal rule of civil procedure, ask if the "federal law" is "on point?" (note that ALL FRCPs are "on point")
If so, apply federal law.
If not, figure out if the law is procedural or substantive... Courts look at "outcome determinative, balance of interests, and forum shopping". If any of those is likely to happen, apply the state law. If not, apply the federal law.