I havent seen that ep yet but Im guessing that explains why Ben looked a little worried and surprised when Jack said he had a letter from Locke on the plane.
I suppose locke could have written a letter after he was strangled to death.
or maybe he guessed? the letter might have said something like.......
Dear Jack,
"I think that ben will stop me from committing sucide then when i mention that im looking for Eloise Hawking he will kill me!"
what the letter did say was......
"I wish you would have believed me."